I haven't seen evidence that ball pythons under age 3 can actually get fat if offered food when they want it--they just seem to grow faster. They can get plump, but not actually obese. I've only seen obesity with adults (cutting back prey size and feeding frequency for a few months solves it).

I'd feed every 5 days. I feed all my holdbacks every 5 days until they're 3, if they'll accept it. (Some insist on switching to every 7 days between age 1 and 2).

I've only had one female slug out on me during a breeding season, and she was a huge old female that threw 13 egg clutches. Only happened once.

Vets will pretty much universally insist on seeing an animal before they'll run tests. They just want to give it a general checkup and enter it in the system before treating it for anything, understandably. Fecal checks here aren't expensive. You're best off finding an experienced herp vet. ARAV.org will help.

You want the vet to do a fecal float, not just a smear. They need to identify what types of parasites (if any) are present, because different types of parasites are treated best with different drugs.