Question 1

If I am defending and use Elusiveness to have the attacker reroll one attack dice, do I loss my Stealth Device?

Question 2

I do have to use Elusiveness before rolling my defensive dice?

Elusiveness

When defending, you may receive 1 stress token to choose 1 attack die. The attacker must reroll that die. If you have at least 1 stress token, you cannot use this ability.

Stealth Device

Increase your agility value by 1. If you are hit by an attack, discard this card

Q1: No. All die manipulation occurs before damage is resolved. Also, see Q2.

Q2: Attacker's dice are fully manipulated before defender rolls.

Thank you

Attack roll - defender modifies / attacker modifies

Defense roll - attacker modifies / defender modifies

Then you compare results.

To tack on an answer to a question that was not asked, but I suspect may be underlying your first question:

To lose Stealth Device you must be hit by an attack. This does not mean that the attacker rolled a [hit] result on the attack roll. It means that, after canceling hits with evades, there are remaining uncanceled [hit] or [crit] results.

I am having a hard time picking my squad and this helps me a lot, again thank you all for the help.