Heh, I never noticed that....
and there's nothing stopping Pre Vizsla being in a Mereel or Fett squad...
this is a good question and food for thought, so here's my humble perspective.
I think Jaster Mereel is not part of the Death Watch, but rather is a 'True Mandalorian' faction,
and so wouldn't count as a follower.
Also, Jaster has a CE, so he can't benefit from CE's (unless they explicitly contradict the rulebook). Followers are characters who do not have CE's. Droid characters cannot benefit from CE's (unless the CE explicitly states otherwise) and are immune from critical hits.
But, if there was ever a Montross... then you'd have something to worry about.
Though, deathwatch faction pieces between Jaster and Pre Vizsla is more of the hypothetical we're exploring eh?
Flipside of the coin, hypothetically 'official' rules notwithstanding,
I think it would work a little like Nom Bomb squads, and a little like the old Rebel Storm Vader squads. That said, yeah... you could have it resolve.
I don't know when you'd have to declare the intention to re-roll... it says once per turn, for any 1 attack a follower in 6 makes. each follower can reroll 1 attack, and if the result of an attack is a miss (so, a natural 1 or does not exceed the DF of the target, as opposed to 'does not cause damage').
Most of the time you'd probably want to reroll an attack you're making on that turn, rather than reroll a 'Mandalorian Death Riposte' attack.
Can a character be defeated again if they're already defeated?
Does Self Destruct 20 only trigger when HPs are reduced to zero (is that what defeat means as opposed to 'removed from play')?
Okay, so long story short
- can the deathshot granted by Jaster be rerolled via the PV CE?: yes, it could be.
- what would happen if the reroll is a miss... : 1 explosion occurs total, because a character cannot be defeated after it has been defeated (which was the trigger for Jaster's CE). this is because of how _Nickname_ explained how triggers resolve. (though it would be cool to see a few super explosions, with Pawn of the Darkside
)