To the best of my knowledge:
- Every conversion counts as a separate hit, which you can see in the combat log when, for example, [Player deals X Physical, Y Fire, Z light damage] and etc. This is why Elemental Fury + Raze is so cheesy, since Raze procs off each of these separate instances of damage, as you said.
- Gravity skills don't "convert" anything, I think they just hit twice if you bash dudes into walls (or into other dudes, depending on the skill). The original hit is the "real" hit, dealing phys damage, and hitting a wall procs a second hit for more physical damage. I'm unclear whether or not hitting the wall procs things like Seal Fate or Disintegration, but I expect that they would. Logic dictates that the wall would be the dealer of said damage, but I doubt the code is set up to support that so it's overwhelmingly likely that this extra damage originates from the player character - meaning that, yes, it probably would proc Seal Fate or Disintegrate.
- I don't know whether they roll to crit or not. Some attacks just don't without ever telling the player whether or not it can. Some talents explicitly don't crit and tell you as much. You'd have to go code diving to find this out, most likely.
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is there a way to know, without going into code, when a talent applies an extra hit and when it just converts or increases some of the existing damage?
The talent should tell you in the description, or you should be able to deduce it from the combat log (maybe). If that doesn't answer your question, then I'm not sure I understand the question.