You shouldn't handle the snake right after eating, especially boids. We do with our corns, but we know they are completely non-agressive, have never bitten, and I'm pretty confident never will. But, even in the case of your boa, leave them in the feed container for about an hour. They will be fine after that.
For question 2, going by the lump can be ok, but can be deceiving as well. I guess it depends on the size of the prey and age of the snake. At a couple of months old, every 10 days should suffice.
- Ken LePage