The stunted growth makes a diagnosis (not that I am qualified to do that) more difficult.
If you have a good reptile vet, I would drop off a fresh poop sample and let them run it. See if that might be an issue.
I adopted an adult corn who was very thin and didn't gain weight easily. He had been underfed and fed live. He had parasites and as soon we cleared that up (which was pretty easy), he started packing on the weight.
Not saying that Corny has parasites, but if he does, they are usually easy to treat.
At 300G, one 25-30G mouse every 7 days should fine. However, as said, if you want to feed two 20G mice, and that works for him, go for it. The issue with obesity is generally when they stop growing and you want them to stay lean. Figment is about 625G and about 5 feet and 5 years old (almost). I feed every two weeks (a jumbo 35-40G mouse) and he maintains. Once a month I feed two weeks in a row, or there about. He started getting little chubby on every week.
Sounds like you are doing really good things and giving a great home to Corny. Don't punish yourself for how he was treated before he got to you.
Keep us in the loop.
David