Interesting subject.
First of all, what is the "Magick" the Nightsisters use ?
Magic was a catch-all term referring to any of a number of supernatural techniques or occult teachings outside of conventional Force powers. It was thought that magic allowed sorcerous practitioners to conjure and wield great powers or to manipulate aspects of reality in order to influence events.
What's more :
The exact nature of magic remains in dispute among philosophers, as does its relationship to the Force proper. It is possible that all "magic" traditions were simply different methods of accessing the same Force, or were emanations of different aspects of it. More so than the Force, magic potency could be trapped in artifacts or locations—but again, this may be just due to different methods of accessing and using the Force.
So we can speculate that the magic Mother Talzin use is but a different way of understanding and using the Force. Can the Force grants ability to others if carefully manipulated ?
We can think of Darth Plagueis the Wise who wanted to create a being entirely made from the Force. Could Mother Talzin achieved what he could not? I don't think so, she didn't create a new being but it is quiet possible she merely modified him by using the Magic/Force and by manipulating midi-chlorians.
Legend
According to legend and the page of Darth Maul on Wookieepedia:
Maul, the Dathomirian Zabrak male who would become the feared Darth Maul, was born on Dathomir to Kycina, a Human Nightsister of Mother Talzin's coven and a Zabrak father who, by tradition, was murdered after Maul's conception. By birth, Maul was a Nightbrother; a clan of male Zabraks from Dathomir who served as warriors and slaves of the dominant Nightsisters. Like the rest of his kin, Maul was subjected, shortly after birth, to an initiation ritual during which he was immersed in a "magic" oily bath, and covered head to toe in black and red tattoos. Later stripped of his black Nightbrother markings, Maul was left solely red-skinned. Agonizing Sith tattoos were applied personally by Darth Sidious in an attempt to cleanse Maul's mind as well as demonstrating Maul's complete dedication to the dark side.
About Savage:
Subjected to the dark magicks of the Fanged God, Opress was granted greater height and size, a full crown of vestigial horns, and fearsome power: he gained even more brutality, an unmatched ferocity, and heightened physical potential and Force abilities. In addition, the Nightsisters placed him under their control so that Ventress could dictate his actions via a spell of loyalty.
The "magic oily bath" mentionned for Maul apparently refers to the Spirit Ichor, the base of all magic employed by the Nightsisters. So we know that in Legend Maul was affected by the magic of the Nightsisters, did it grant him all of his powers ? We don't know, maybe he was already force sensitive and the magic only gave him a "boost".
The process Savage went through seems to be the hardcore version of what Maul got in his childhood and you add with that some mind controlling spell and body manipulation.
Conclusion in legend:
- Is Savage truly Force sensitive? Or did his Force powers come solely from Mother Talzin's "Magicks"?
With what we speculated earlier, we can conclude that Savage is indeed Force sensitive but there are strong evidences that his sensitiveness was granted with magic by giving him more midi-chlorians. So both of your questions can be answred by a "Yes".
- Likewise, is there any indication that the Force powers that Maul (a.k.a Darth Maul) possesses from Mother Talzin, or naturally occurring ?
Like I said earlier, we don't know. Maybe he had some before, maybe not. But seeing as he is powerful as a Sith, I think he had more midi-chlorians than the average Zebrak
Canon
About Maul:
The boy who would become known as Darth Maul was born the son of Mother Talzin on Dathomir. [...] When Maul was a child, Mother Talzin became an ally of Darth Sidious, the Dark Lord of the Sith, and the two exchanged wisdom and power. Sidious eventually abandoned Talzin, despite promising to make her his apprentice, and instead took Maul once he realized the boy's potential. Once under Sidious' tutelage, Maul began training as his Sith apprentice.
About Savage Opress:
In accordance with their plot against Dooku, Mother Talzin and her Nightsister witches employed their magicks to grant Opress fearsome abilities, placing him under their control.
In canon, the story about Savage seems the same as in Legends but there is no mention of Maul being altered by any means.
Maul seems to have been Force sensitive all along and Savage only altered as a mean of revenge, that is why Maul has a normal height.
Conclusion for Canon:
- Is Savage truly Force sensitive? Or did his Force powers come solely from Mother Talzin's "Magicks"?
Same as Legends, altered by magic, granting him body modification and Force sensitiveness.
- Likewise, is there any indication that the Force powers that Maul (a.k.a Darth Maul) possesses from Mother Talzin, or naturally occurring ?
It seems that in Canon, Maul is a natural, no alteration as far as we know.
