Yes, a small lump is fine - just not a significant one. The rat should be about as big around as your boa, but if it leaves a tiny lump, it's fine.
I mean absolutely no offense, but I think you're overthinking this a tiny bit. The precise size of the rat isn't all that important as long as you're not offering something huge. I don't weigh my rats at all, I simply eyeball it. Once in a while, it leaves a lump slightly larger than I'd prefer, but I just give something a tad smaller the next time around. Boas don't weigh their rats in the wild, lol.....
I personally feed f/t so I would offer pre-killed, but as long as you are observant and never leave a rat in with your snake unattended, you can feed live.
Also, at a year old, I would feed every 10 days, not every 2 weeks. 2 weeks is a better schedule for a 2 year old snake. But again, this isn't set in stone.![]()