GreatFuckability
I'd got all my offers but one by the time UCAS applications closed in the January. I was the last year where the universities you applied to could see where else you'd applied & if you'd been accepted or rejected so as soon as Cambridge accepted me Bristol rejected me. We'd been told (my school's "support" for people wanting to go to uni full stop was abysmal, for Oxbridge it was so bad as to be damaging...) we were the first year where they wouldn't be able to see this information or I'd not have bothered applying!
I have a deep & abiding affection for York Uni because they offered me a place within a couple of days of submitting. I felt really guilty for not putting them as my insurance choice after that...
Ooh look, there is the actual point of the thread all the way over there...
Am assuming OP had a moment of daft & wrote AS instead of A2. Rather than claiming her DD's done her UCAS application at the start of Y12.
Grandmother didn't exactly cover herself with glory with her comments. Did she offer to pay for your DD to attend these amazing schools & are the comments thus more aimed at [blaming] you [for "depriving" her]? What sort of school does your daughter go to?
If she wants to go to Oxford, I'm afraid her grandmother is right & A's may not cut it, depending on the subject - you need at least one A to do a science subject; & 3 to read Maths, Maths & Philosophy, or Maths & Statistics. (Was a bit shocked it's not required to read arts subjects TBH as Cambridge's standard offer changed from AAB to AAA when the grade was introduced & has now gone up to A A A in most subjects & A A A in some of the Sciences...). Again, depending on the subject she wishes to read, there may be other universities that wouldn't accept her without at least one A. Clearly your MIL had no business saying what she did but it may be that she is genuinely concerned & trying, HUGELY misguidedly, to give your daughter a "wake-up call" because she's honestly convinced her future hangs in the balance here.
If you look at what she said, it wasn't that she was too stupid to go there, it was all the "work harder, buck your ideas up" stuff of someone who thinks they know what's gone wrong & how to fix it. (The bit about wasting money on uni open days being a way of rubbing in her "you need to change what you're doing or you'll not get anywhere" line.)
Again, it wasn't her place to say it & what she said was hurtful. However, I don't think it was in the least bit acceptable for her to tell her grandmother to fuck off. What was said wasn't nice, but nor was it a torrent of abuse. Your DD could have disengaged by just hanging up. Also rude, but if she'd said "bye now granny", barely rude at all. And TBH it doesn't matter your MIL was rude first & that MIL is older etc - when she was wee you didn't teach your DD it was ok to bite back when the chompy child at toddler group sank their teeth in: this wasn't a time to be biting back (albeit less literally) either...
Your DD telling your MIL to fuck off will just have given MIL something else to think she needs to worry about (i.e. your DD's attitude).
Given you think your MIL can be "a complete cow" & that she's a bee in her bonnet about where/how your DD is educated, it probably wasn't the wisest move to have your DD tell her her results herself...
Anyway, as to whether you're being unreasonable OP, I think that yes, YABU - & I don't really understand what you're hoping to achieve, either. Have you banned your DH from talking to your MIL about UCAS? What does your DD think about this? Surely being able to tell her that your DD got an offer straight away would have been a good way to show her that she's doing fine despite not being at boarding school? It seems a terribly petty way to drag it all out & potentially create a massive family row completely unnecessarily.