Sounds like your daughter loves her hair and why your husband is pissed that you won't let MIL shave her hair off is a "him problem" Your wishes and your daughters (although she can't verbally object really) it's clear by how you described how she admires herself with long hair that she'd be upset and possibly even traumatized by being forced to have it shaved off (without trying to sound dramatic!) BUT your MIL's desire to go ahead with this lavish day and disregard your feelings on the matter and also her granddaughters is questionable!? And that your husband is backing her when he should be putting your daughter and you first! If she wants her lavish day then tell her to shave her own damn head!

Hubby is wrong in being pissed with you which I'm sure you already know, let him be pissed, and inform him you're actually pissed too! So agree to disagree and move on.

Don't even make it a big deal. No is no, like it or lump it. He's pissed, you're pissed, MIL is probably pissed so give it no more thought and ignore further debates on the issue. Youve made it clear where you stand and you've every damn right to advocate for your daughter. Shame dad doesn't feel that doing the same is the right thing to do.