So sorry for your cat losses. So even after 20 years I can look forward to unexlained illness?
But wow.. the number of butterflies you helped put in the world must be amazing.
I am pretty new to this but I had several unexplained deaths this season too. Cats were walking off their food,expelling green fluid and then dying. I also had one that did not grow and shed as they others in "his" batch did. By the time he should have been a chysalis he still only looked like a 2d or 3d instar! Then he too finally died.
One thing you said.. about the cat hanging in a V....reminds me of something I read about Nuclear polyhedrosis virus ,but I can not find the reference.
I hope you find a more conclusive answer.
Not knowing ( and not knowing if it can be prevented) drives me crazy,too!
Maybe Jim or Scott will have an answer for you!