So, still no idea really. I wrote it all down so here goes...
Bloods all came back perfectly normal. No indication of anything, other than stress.
There's definitely a "big inflammation event" but it's possible that what the vet saw in Ella's organs was slightly outside the norms for dogs in general but perhaps normal for Ella. They can't be sure but the bloods indicate that everything is ok.
For now they just treat as a case of severe acute gastritis. Treatment includes pain relief, fluids and antibiotics. They've had her walking a little but she's sleeping for now and they want her to stay overnight.
They will call in the morning and hopefully we can pick her up then with oral ABs and pain relief for at home.
We will follow up with our vet at some stage in the next couple months, assuming no repeat episodes inbetween.
Positives - everything they investigate and test comes back clear. Less likely to be a chronic issue as, if we were getting these symptoms from the issue, they would have expected her bloods to show it.
Negatives - I can't help but feel like she's been through a huge, invasive procedure that wasn't actually needed. Part of me wishes I'd just told them to do bloods and put her on a drip as the outcome would have been the same. Hindsight is wonderful though. Also, we still don't have a definite answer so we don't actually know what's made her so sick.