NTA.

Everything was fine until they intentionally spilled wine on your dress. Yeah, maybe they were wrong for thinking the dress was inappropriate, but everyone is entitled to have and keep their wrong opinions to themselves. Their 'calling you out' at the event (of course you don't have another dress with you - who brings a backup full outfit to a wedding?) was wrong, but a mild transgression on their part. When they spilled wine on your dress, that is absolutely far across the line, and you should tell them you expect them to pay for the dress. Sue them if they don't.

I saw A giggle and M grin

Sounds like your cousin married an asshole.

I would expect a full apology from M specifically (it's his and A's event, and him encouraging them damaging your stuff is not acceptable), and I would expect M's family to pay for the dress (unless you can get the wine stains out, but... good luck).

It's a little bit unfortunate that your gift was to A and M, and it's M's family who are being horrid. Taking the gift is harming A and M, when they didn't specifically do the bad thing. But, M grinning at your misfortune, to me, comes across as tacit approval of his family's actions, which then tips your taking the gift back into 'acceptable' territory to me.

I think the fair outcome here would be for M and his family to apologize, and pay for any damage. That is the bare minimum, and doesn't undo their horrible action. That is literally the minimum thing they need to do in order for there to be any possibility of going beyond this.

I think A should express condolences to you that this happened to you at her event, and that she should be mortified at her husband's and his family's behavior.

Once those are done, if you believe the apologies are sincere, then I would consider getting a normal wedding present for them after the fact and having it shipped to them. Something closer to $100-150 price.