What if you got another buddy for Nibblets? I'm sure you've already thought of that though and have a good reason not to.

Don't rats usually live 3-4 years? With her tumors, maybe you are just helping her to the bridge before she has too much pain. I don't know a lot about rats, but it seems that you have thought extensively about this so I trust your instincts.

I am very, very sorry to hear about Gillian too.