They are professionals, I am sure they can put the disapointment behind quickly. Palat also came to WCH in 2019, when he was in similar situation with Tampa. I think this should not be such a problem.
I see the chances of the guys coming as follows:
Pasta 65-70% - Sure he must be extremely dissapointed not bringing home the cup after the amazing season he had, but he is also a big patriot a loyal heart and he generaly seems like a guy, who is just always hungry for more hockey. I believe, that if he is healthy he will come, but if he says no, its completely understandable and no one can say a word against it. He always come through for national team so far, so he has every right to just rest if he wants to.
Krejci 10% - without contract and he was injured during the series, so its very unlikely he will come. I give him 10% cause as you said, he is a 37yrs old veteran and a Boston legend and he is in a position, where he can just say "screw it, I dont care where I play next year, I just want to play with Pasta at WCH lol." But as I said its very unlikely.
Zacha 50% - He literally has no reason not to come this year lol, but we all know how it is with him and national team, so I dont hope for anything. I guess, that if Pasta comes, Zacha will too, but if not, then its really a 50/50 with him.
Zboril 80% - I am sure he will want to go, but the problem is if there will be an interest from our coach. And even if they want him, I am sure he will come to play in Brno and he will have to earn his spot in the final lineup.
Nosek 10% - Without contract, he probably wont want to risk it, but you never know.
Lauko 0% - I am sure he will be send back to Providence for AHL playoffs.
Francouz 50% - I really want him, he could be a really good duo with Vejmelka. but he was injured a lot also during ther season, so I dont know if the club will even release him.
As for the AHL guys I guess this is it and no one will come anymore sadly

(maybe Galvas). The last match for Rochester is on 7.5. so Kulich and Rousek wont be available for Brno and I am pretty sure Kari wont take them straight to WCH. Same for Lauko and I worry also Galvas (althoug there is a small chance).
Man that AHL playoff format is so stupid lol, such a ridiculous gaps between the games ...