I'd lean towards this. It goes with the starting issue. But also with dieseling (not related to the starting problem) an easy check is your idle, too much fuel at idle can cause dieseling on a hot engine. Check your A/F mixture and speed.
Was the starter new or old/used? I haven't noticed heat soak being a big problem with exhaust manifolds, usually when headers are put on. I don't think I've ever had a heat shield in place on any of my rigs. Just strange there's a lot of seemingly not-related issues. If it starts or cranks fine when jumped, but a different battery has no effect I dunno? lol freshly painted engine, did you scrape all your grounds? But that doesn't fit with it starting fine when cold.