I think the difficulty is, op, that age 60 and size 18-20 is not, in itself a look. There's colouring, body shape and above all, individual taste and style to factor in, even if her style is not to give a shit.
Fair enough, you don't want to talk about the politics, nevertheless it sounds like there's a lot of family politics behind your op. I'm trying not to go wading in when I don't know the waters but, unless there are health factors or 24/7 care responsibilities involved then I think delegating the choice of what to wear to your daughter's wedding to you - especially if you're also the bride - is quite odd. You're in a no-win situation and I really sympathise.
Can she not do at least some of the work here, even if it's only looking at some magazines and Pinterest pages? Because if she won't then frankly I'd say stuff it and just get her the dress in your op. It sounds like you have enough to do as it is.