I'm pretty sure when you use a card, once it's started it doesn't matter what the outcome of any part of it is, all parts of it continue to resolve (an example of this could be Vader, it doesn't matter if the 2 damage destroys your ship, the critical damage still resolves). So with Ellusiveness "assign a stress to your ship" and "causes the attacker to reroll an attack" all that matters is you could generate a stress, if that is the case you can play the card, and both parts of it resolve independently.
The reason that Soontir's ability doesn't work is it's conditional on him receiving the stress not the stress being generated.
Edited by Rodent Mastermind