From what I understood, paradoxes seem to be relatively harmless in the Arrowverse. What happens exactly, to allow the abuse of time remnants, is that the speedster needs a very strong will and a good knowledge of how short-time travel works.
If a speedsters goes back in time, it changes the pastfor everyone except themselves, which are pretty much unaffected because of some speed force explanation (or, as some might say, deus ex machina).
I'll explain with an example.
Barry is running. Barry knows that he needs to be "two" of himself to beat whatever is to come, and one of those will need to sacrifice himself. The death is probably required even without needing a sacrifice because I would expect more problems to show up if there are two barrys for an extended period of time.
So Barry tells himself. In exactly five seconds, if another me shows up from the future, I'll team up with him and do part A of the plan, and sacrifice myself shortly after. If no other me shows up, then I'll waiut five more seconds, travel back in time 5 seconds, and do Part B of the plan, while letting the past me do the part A and die.
Barry needs incredible resolve to do this, because he has now way to know which Barry he will be (A or B), and once he knows, if he ends up being A, he can't change his mind and has to accept his own death. Only if he gathers that resolve to act differently depending on the time travel, does the time remnant happen. When he time travels by accident, like in Out of Time or Legends of Yesterday, then he simply replaces younger Barry, because that one did not have the resolve to die for the plan, so can't continue to exist.
Everything is decided by the speedster's will to play either halves of the plan even without knowing which half he'll have to play and having to die in one of the cases.
It is actually easier to do for an "evil" speedster like Zoom, because instead of sacrificing himself in plan A, he kills Zoom A in plan B, so he needs a lot less willpower to do that, because Zoom A wouldn't have the option to back off after initiating it, since he will get murdered. Not having that option means he doesn't need as much will to actually go through with dying.