Don't for a minute think that you are to blame for any of this. And don't think that the dental was the bane of his existence, either... dentals, especially in older/sick pets, are always a good idea. Walking around with bad teeth or a mouth full of bacteria is just asking for issues (bad mouths can lead to cardiac/respiratory disease, sepsis, etc). The fact that he hasn't had a nice recovery kind of indicates that something deeper was going on to begin with. But don't you blame yourself for a single second!

Hoping for better news...