Right, I’m probably going to regret this as it’s obviously a pretty heated situation but……
If I was presented with a dog with the signs it sounds like your dog showed (Nystagmus, head tilt, circling?), a “stroke” would be right at the very top of a differential list. Yes middle ear disease can show similar signs but rarely is it that acute in onset and usually there are other signs – pain/swelling round the ear, discharge, temperature, general signs of illness etc. Yes it’s still a possible differential without this but I would think a very unlikely one.
I can’t really comment on the decision to euthanase her but I can’t imagine it was one taken lightly, or without serious discussion with your parents. I don’t think heavy sedation would have been in her best interest either as that could have simply made the situation worse. The fact they decided to put her to sleep rather than leave her to see if things settled suggests to me that the signs must have been quite severe or there were other complicating factors? I know it doesn’t make things any easier but, given her age, surely there are some positives to be had from the fact you didn’t come back to a severely ill, suffering animal and can remember her as she was – a fit healthy friend?
I’m very sorry for your loss but any decision for euthanasia in veterinary practice has to be made with the animal’s best interest in mind