A lone bun doesn’t necessarily show hormonal signs. It’s different when there are more pets in the household.
We have one member whose girl bun started to show hormonal behavior at around 2 years. She was spayed then and was found to have pathological changes to her reproductive organs. She was all good after the spay though. It is however recommended you spay girls younger, because the surgery is less complicated when the girl is about 6 months to a year old.
Were Muffin and Munchie from the same litter?
Even if vets do make blood tests, there’s no guarantee a bun will survive anesthesia/surgery. Some results from a blood test can however show that the bun shouldn’t be put under (kidney and liver function tests f ex), but even if a bun’s tests come back excellent, there’s still never any guarantee.
I’m very sorry you lost Mubchie, I remember it of course. I can see how you’re scared to have Muffin undergo any procedure.