This is definitely "Out of this World" (1964) by Alfred Bester. It was published in his 1964 collection The Dark Side of the Earth.
Howard Campbell introduces himself as someone who is faithful to his wife, but has a bit of a roving eye and will "fall in love" with random women he meets. In particular he tends to build fantasies about ...
It is The Repairman by Harry Harrison
This department is officially called Maintenance and Repair, when it
really should be called trouble-shooting. Hyperspace beacons are made
to last forever -- or damn close to it. When one of them breaks down,
it is never an accident and repairing the thing is never a matter of
just plugging in a new part.
This sounds very much like a quote from what was one of the seminal sf works of its time, and one of my personal favourites, if a little cheesy by today's standards.
The movie I'm thinking of is Forbidden Planet, from 1956.
The actual quote from the movie is:
I'll bet any quantum mechanic in the service would give the rest of his life to fool around with ...
This is surely the BFG (The Big Friendly Giant), an animated film released in 1989 based on the children's story by Roald Dahl.
The older giant takes care of a young orphan girl named Sophie, and takes her from her orphanage to the land of the giants where he lives. There he protects her from the other ugly, brutal giants who live there, who have names like &...
Year Zero by Jeff Long.
The scientist is Edward Cavendish:
The occassion of the first Congressional hearing had been Cavendish’s infamous “meat tree.” Funded by Burger King, working in a private lab in Nebraska, Cavendish had conjured up a herd of headless cows. As a matter of fact, Cavendish’s cows did have heads, but genetically stripped to the basics, a ...
This is almost certainly "The Riddle of the Trumpalar Tree" by Judy Bernard-Waite (goodreads link).
Australian, the description on Goodreads doesn't specify which city
Main characters are 11 year old twins
petition to save an old Moreton Bay fig tree
the main characters 'enter' the tree, and meet its guardian
the tree does, indeed, have a ...
This sounds like Puck of Pook's Hill by Rudyard Kipling. The book contains a series of linked stories about the adventures of two children across different times in England's history, guided by a magical elf. One of the stories definitely has an encounter with a Roman soldier which leaves the children impressed by the similarities between the Roman and ...
Pretty sure this is Year Zero (2002) by Jeff Long.
The Christian relic, the plague, and the clones -- one of which claims to be Jesus Christ -- are all mentioned in the Goodreads synopsis:
An archaeological manhunt is raging in the holy land -- a hunt for the historical Jesus. For Nathan Lee Swift, a young American field researcher and expectant father, the ...
This is likely only a partial match.
Could this be The Magic Treehouse #13: vacation under the volcano?
In this story, the protagonists Jack and his sister Annie are taken to ancient Pompeii, just before the eruption of Mt Vesuvius in AD 79. I think they have to find an ancient library before it is destroyed by the pyroclastic flow from the eruption. ...
Douglas Hill - Huntsman, Warriors of the Wasteland, and Alien Citadel.
The Alien race is the slavers; they capture the protagonist (Finn) family, and in the course of trying to rescue them ends up saving the world.
Baer is from a race of servants to the Slavers, but is a bit of an independent thinker and joins Finn.
Some relevant quotes from the first novel (...
The Veteran series by Gavin G. Smith. It is series of two books, which matches your description. The first book is Veteran and the second is War in Heaven.
The protagonist is Jakob Douglas, and he is ex-special forces rather than ex-SAS. As you say, his augmentations were disabled when he left the special forces and he is now doing various security work and ...
Jeremy and the Puddle by Christopher Hobbs? It was published in 1990, and involves a child falling into a puddle (without his mother) and entering the world of Reflections. He does make his way back home safely.
Jeremy is an awful child. He finds a puddle, jumps into the middle and disappears. He finds himself in the watery world of Reflections ruled by the ...
This is Supa Strikas (2008–2015).
Supa Strikas is an animated television series about the super league's greatest football team based on the pan-African football (soccer) themed comic of the same name, which is produced in Malaysia by Animasia Studio, and Strika Entertainment originally in South Africa.
In their bid to raise the Super League trophy (...
Pretty sure this is The Miracle in Valby (1989).
The Miracle in Valby (Swedish: Miraklet i Valby) is a 1989 Danish-Swedish drama film written and directed by Åke Sandgren, and starring Jakob Katz, Troels Asmussen, Lina Englund, and Amalie Ihle Alstrup. The film was produced by Nordisk Film, won the awards for Best Film, Best Director and Best Screenplay at ...
The green skinned K’ahari attacking Terran colonists is in the story Jean Dupres by Gordon R. Dickson. I read it in the anthology Nova 1 edited by Harry Harrison.
For there were two of the K’ahari young men standing just inside the edge of the small clearing about twenty feet off. They must have been there before I stepped through the last ferns myself, ...
I'm 95% sure you're talking about Independence War 2: Edge of Chaos.
Sadly I never got to play that game much. As I recall some mission was bugged for me and I couldn't progress. So I never saw the ending. But I do remember the beginning of the game as you described it.
The main character's dad is killed by some wealthy corporate boss in the intro. The main ...
This might be Shirayukihime to 7-nin no Shuujin, aka Snow White and Seven Dwarfs.
The main character, Shakudou Takeru, wakes up in the collapsed ruins of Tokyo. It's only two years in the future, not quite a full on post-apocalypse. Tokyo has become an island after multiple meteor strikes, and the city government is in charge, with officers in military ...
This is Masterminds by Gordon Korman.
Eli Frieden has never left Serenity, New Mexico...why would he ever
want to? Then one day, he bikes to the edge of the city limits and
something so crazy and unexpected happens, it changes everything.
Eli convinces his friends to help him investigate further, and soon it
becomes clear that nothing is as it seems in ...
This is Not For Glory by Joel Rosenberg.
The Inspector General is Tetsuo Hanavi, who narrates the story. His rank of Inspector General in the Metzadan Mercenary Corps is really just a cover that allows him to travel more freely; his real job is assassin for the Metzadan government.
Tetsuo is sent to find (and kill) his uncle, ex-general Shimon Bar-El, who ...
Could it possibly be NM Browne's Warriors of Alavna?
Dan and Ursula are transported back to c75AD. Dan acquires brilliant fighting skills and Ursula accquires magical powers and becomes a Priestess. Mostly though they are in the company of Britons still trying to resist Rome.
If you read these online, this might be the Firestaff series, which begins with The Tower of Sorcery:
Tarrin Kael is a farmboy from a small village with dreams of becoming one of the famed Knights of Karas. Unlike most of those who have such aspirations, he’s got enough training to hope he has a decent shot: from his father who used to be a Ranger, and from ...
The Congress (2013).
Robin Wright plays a fictionalized version of herself as an aging actress with a reputation for being fickle and unreliable, so much so that nobody is willing to offer her roles. [...]
Robin's longtime agent Al (Harvey Keitel) takes her to met Jeff Green (Danny Huston), a CEO of the film production company, Miramount ...
Might this be Jeon Woo-chi (2009) AKA Woochi: the Demonslayer...?
I don't think the hero rides a scooter in this, but it seems to match pretty much everything else you described.
Jeon Woo-chi, an undisciplined womanizing Taoist from the Joseon era, ends up in present day Korea causing mayhem with his knack for magic.
A bit of a long shot as it does sound like you are talking about a straight-up story, but it couldn't possibly be a puzzle book at all?
Time Train to Ancient Rome, by Gaby Waters (Part of the Usborne Puzzle Adventure Series) bears a number of similarities to your description, though admittedly transports the protagonists to Rome itself rather than Roman ...
Is this the video you were referring to...?
If so, aside from Double Dragon, I saw only one other Sega Master System game mentioned with 'Dragon' in the title, and that was Dragon: The Bruce Lee Story.
Dragon: The Bruce Lee Story is a Sega Master System and Sega Game Gear platform game based on the 1993 Dragon: The Bruce Lee ...
Could you be thinking of Foot 2 Rue, aka Street Football?
Foot 2 Rue takes place in Port-Marie, a thinly-veiled representation of Marseille. The kids of Port Marie organize a Street Football World Cup with the help of a former professional footballer, thus attracting players from all around the world and becoming famous worldwide in the process. The sport ...
This is the Regeneration series by L.J. Singleton.
Book 1: Regeneration (2000)
Book 2: The Search (2000)
Book 3: The Truth (2000)
Book 4: The Imposter (2000)
Book 5: The Killer (2001)
The synopsis on the Google Books page for the first book notes that three scientists created five cloned children, and that two of the scientists had to save the clones from ...
Might this be Challenge of the Five Realms: Spellbound in the World of Nhagardia?
Challenge of the Five Realms is a plot-heavy axonometric title set on the flat world of Nhagardia. A prince must race against a creeping darkness to retrieve the five crowns of Nhagardia's five kingdoms (human, gnome, elf, fish-man, and effete flying man), whose leaders have ...
The channel is Existence is Terrifying by exurb1a
I've found a third possibility - a book in the Questers series by E.W. Hildick.
From the Goodreads page for the first book:
Peter, who is bedridden, has a telephone and a remote-control television unit on his bedside table—and if the Questers’ Master Plan works, he’ll have a walkie-talkie set that will keep him in touch with his friends’ activities. Andy ...