The source novel indicates that it's simply the height of summer in the UK, noting that 1995 was indeed a surprisingly hot summer in real life.
No additional explanation (e.g. that it's magically hot) is offered by the author.
The hottest day of the summer so far was drawing to a close and a drowsy silence lay over the large, square houses of Privet ...
The "terrifying truth" is the fact that Voldemort returned, which Fudge is shown to deny in the part immediately before:
[Kingsley shows Harry a newspaper with the headline "The Boy who Lies?"]
He's been attacking Dumbledore as well. Fudge is using all his power, including his influence at the Daily Prophet, to smear anyone who ...
Your notes and memory appear to be correct. A text search for "Dobby" in the e-book version turns up active results for the following:
Chapter Two "Dobby's Warning"
Chapter Ten "The Rogue Bludger"
Chapter Eighteen "Dobby's Reward"
Dobby is also passively referenced in the following chapters:
Chapter Three "The ...
The only crimes we see Tom Riddle do while in Hogwarts are releasing the basilisk from the Chamber of Secrets and killing Myrtle. As these things both happened in the girl's bathroom, there weren't any portraits there to see him do those actions. Anyway, I'm sure Tom would've thought of portraits seeing any of his crimes (and for that matter any students in ...