Are we just establishing what we know so far?
If so that about covers it, although I think it is worth also mentioning the WOJ about the multiverse.
I.e. that EVERY time a choice is made (by a mortal), a split (exactly like the branches created at the creation of a paradox) is made diverging into two timelines: one where the choice was made, one where it wasn't made.
So one imagines that plays into how I person COULD potentially alter their own timeline. From my limited understanding of physics, in the many-world theory, a person who went back to change their own timeline (for example like Bob's example of the person who warns another of their car being stolen) wouldn't cease to exist as such. Rather the person would continue in one timeline where they had succeeded in warning the person so their car wasn't stolen, and one where they failed and the car was still stolen. But in the first branch they would still have to go back in time, which would either create a time-loop or they would no longer be in sync with their own timeline. They would effectively be inhabiting someone else's universe. Which gets really depressing when you think about the fact all the people they had known would be in their old universe, and while these people would be effectively the same they wouldn't ACTUALLY be the same (unless I miss the mark, or Jim has another explanation).
Dark is a reasonable tv show about time travel from a physics POV although it has it's own issues, but it is closer than most.
I like your explanation Morris about how Merlin was able to be in five times at once.
The thing to remember is that while we like to separate time and space, from a physics perspective they are incredibly linked and so often we say space-time as the overarching phenomena. To my mind, Merlin Emrys was in the same physical spot. But that spot was made up of five different "layers". Looking at it from anyone Time Point it would merely appear he was in the one spot. But were you able to see all five Time Points, you would see him in all of them at once, making the exact same movements (rather than going to each time separately and working magic). In fact I think this would be the only way to do what he achieved.
Interestingly, it is the state of being I think the Angels of the Dresden Files exist one (to varying degrees - I am unsure of how Denarians experience reality hampered as they are). Uriel doesn't necessarily need one million bodies/avatars (although I suppose he could do it that way) to be in one million different spots in space-time. He could potentially just exist in enough dimensions that it wouldn't be possible to measure him in all those places, limited as we are to 3 (or 4...there is debate). Jim sort of confirms this when he talks about how the Angels only show up as a whispered presence so they don't squish the sandbox, only inserting a tiny part of themselves into a point in reality. Which is probably the case with Ferrovax and the higher immortals, although who knows.
But I am getting off topic here, apologies.