1. there is no evidence to suggest that snakes become cage aggressive due to feeding in their living enclosure, and as you mentioned, it would be tough to do so when s/he gets bigger! some benefits for smaller snakes, but its not necessary

2. mice have a lower tissue:bone ratio, so more of the weight is bone when compared to larger mammals (including rats).
Aaaand, hate to break it to you, but mice will not sustain Oliver. He is going to be eating rats...but I don't have any advice for you in terms of what to do with rats when feeding...I would keep the rats in a different room from the snakes, and definitely scrub down before handling oliver after handling rats