Looking at the Vegas situation with Dadonov, it seems like the deal will get voided, which means Vegas will have to do something else to clear cap space.
Any player they trade at this point can't play for their new team. I originally thought it was just in the playoffs that he couldn't play, but according to Mouser, who is very knowledgeable about these things, the player can't play even in the regular season.
So which team is going to take on Dadonov's contract and then not play him the rest of the year? I can't see any team doing that. He has another year left on a 5 mil cap hit, with 6.5 mil in actual money. Reilly Smith is a pending UFA, and he also has a 5 mil cap hit. Moving him out would clear the same amount of cap space as moving Dadonov. He also doesn't have any trade protection according to CapFriendly.
It would be a really shitty move on Vegas' part to trade him, since he will then be ineligible to play the rest of the year, but I don't know what other choice they have. Martinez is going to come off LTIR eventually and they have to make room for him.
So, it seems like Vegas will have to pay some team to take Smith. The Rangers could take on that contract in exchange for Vegas' 2nd this year, or 1st next year.
It may not end up playing out that way. The league might just penalize the shit out of Vegas and allow them to activate Martinez without first clearing cap space. That wouldn't be an ideal solution as it puts the integrity of the league into question, but it also doesn't screw over a player who has done absolutely nothing wrong. It's also possible that the league makes an exception and allows the traded player to still play for his new team. That would also put the integrity of the league into question, though I think to a lesser degree.
Another option, and the one that is likely most by the rules, would be to have Vegas forfeit every game once Martinez is activated, since they won't be cap compliant. It's a fitting scenario for Vegas, but gifting other teams free wins doesn't seem very fair. Vegas has 16 games left. I don't know when Martinez will have to come off LTIR, and they will likely stretch it as long as possible, but even if it's only 8 games that they forfeit, that's 8 free wins for teams that may be in a playoff race. They play Vancouver 3 times in their last 12 games. If all of those are forfeit, that greatly helps Vancouver's chances of making the playoffs.
It's a huge mess and I don't envy the NHL in figuring it out. Then again, they should have a system in place to ensure that no trade lists are registered with the league, and not just managed by the teams.