You've provided exceptional information and detail. You are also obviously dedicated to this cat and this isn't as common as I'd like to see (we have been rescuing and providing care for cats for decades!).
Anyway - while it's not really common for cats to have heartworm, it's not unheard of. Since the vet is suspecting this anyway, you might want to share this site with him
http://www.vetmed.auburn.edu/distance/cardio/all.html#Clinical%20Signs
It's loaded with professional information and guidance for the medical professional when it comes to diagnosing and treating this problem.
I really hope it helps and would appreciate it if you'd let me know how this goes. I can 'hear' your concern and having been there a few times with medical emergencies and complications, I completely understand how you want to just fix this and make this cat better .
Please check back with me ok?
Good luck and God bless !
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You remain responsible, forever, for what you have tamed.

