Sorry for all the stress this is causing for you.
I think it's important you speak to the specialist who knows all the results of tests that have been done, and have Quinn's full story.
Speculating on the what-if's will just do your head in.
What's important is that Quinn is doing well now. You don't know for sure that it will happen again, but several forum dogs have had colitis problems and although can be a pain, are well manageable. It might not be what you want, but they are manageable with diet and meds.
Anne and Kate have nailed it with the writing everything down bit, I would really recommend you do that.
As a little aside, colitis in humans is now measured with a stool sample measuring something called "faecal calprotectin". Biopsies aren't always that sensitive - if you don't biopsy the right bits they show up as normal.
I really hope you get the answers you need x