Angels are notorious for developing internal parasite issues. What you thought was a nice fat fish may very well have been a bloated ball of internal parasites. I wouldn't replace it, but you should try and find the carcass.
There's no odd-numbering rule of thumb. You just need to keep an eye on how they interact as they grow. Four adult angels in a 75, depending on what combination of M/F they end up being may force you to move one or even two. Bottom line: they are cichlids. Once they pair, it can get a little intense between them and the rest of the fish in the tank. And then some pairs can actually do very well in a community setting. No rule covers all the possibilities but you need to be ready for the worst case.
"Why can't my wife see all of this stuff as an investment?"