You're too late to rebuild trust...but why do it at all?
This is probably an answer you don't want to hear, but you've spent the campaign up until now with Strahd turning the players against each other. That effort isn't going to be simply undone by a spell or so - the players are deeper in this than potentially their characters are.
And this is what being in Barovia should be like! You've more than succeeded at the goal of sowing this distrust - and it's pretty much been done specifically for this fight against Strahd. Don't undo the effort you've done! You and your players have embraced the feel of this campaig - don't let it go now!
But, this also makes you as the DM less than trustworthy as a source for how to 'fix' this - as you were the cause of it! As you've said, even a point-blank Meta-DM statement might not be the solution you think it will be because the players are already thinking you're trying to 'fool' them. The paranoia here is right and good - don't let it go!
But if you really think that the truth must be revealed, then it does shift what you can do to fix it - and it's mostly going to be listening to your players.
When the DM isn't trusted, let the players develop the action
Even more problematic with a lack of trust in the DM is that it will extend to any information the DM provides - whether direct or from a NPC.
Because you are now a 'untrustworthy narrator', your output is under question - which is a great outcome for a campaign like CoS. Leaving the players guessing makes for a very atmospheric Barovia - but as you've noticed it creates some problems, too.
In this case, I've found the solution to be in giving more control to the players in terms of figuring out 'what to do.'
As a DM, I have definitely turned my players around because of scenarios like this that I've developed - and I've learned the only way to fix it is to give the players control again. Let them come up with a reasonable plan, and then have it succeed. That way it is their idea that was used and they can trust in the outcome because they are invested in the idea itself. Heck, even when I'm ham-handed, they still generally miss things.
This has generally worked out for me, but you may need to be more obvious/heavy handed in your approach for resolving this. It is quite possible that this isn't resolved before the fight - and I'm not even sure that it should be. You've worked for this outcome, but now you want to undo it...why?
Leaning into the uncertainty
Let the players start off without trust in the final battle, and then maybe Strahd keeps laughing and mocking about the seeds of distrust he's sown. This is a pretty on-the-nose way to tell the players their paranoia was unjustified and that they were played. I honestly don't think there's a solution where your players will feel comfortable in the knowledge that they're all on the same side before you go into this fight.
And I don't think they should be. You've developed this situation, and wanting to undo it when it reaches the climax seems to be what's causing the strife here. Instead, lean into it and let the players go in without trust - then rebuild their trust as they fight and Strahd "tells his plans" in the way that all megalomaniacs do to their enemies :)
Or, as Guildsbounty said, you can let it play out all the way to the end - and I think this is a stellar solution. You and your players will talk about this for years about what Barovia did to the party and how they didn't know what was really going on until it was over.