You should be mad at all of them. Remind SO that if this relationship is to progress to the next level and milestone, he needs to start considering you as a partner, and by extension FSD as part of his family package. He can't pick and choose when he can switch it off.

MIL was manipulative.

At the very least, they could have discussed it with you like adults and you all could have made the best decision for FSD. That she lied about changing dates is a deliberate act of exclusion.

I would hold off rushing into any more relationship milestones that tie you to SO with red legal tape until you see him be on your team and behave like an equal partner. Don't marry someone until you see that potential in them otherwise you will always be the outsider. This is a big one that needs resolving before, not during or after marriage.