It's never too late, it just takes longer to earn the snakes trust. Regular handling will curb the behaviour over time. Just needs to be built up to avoid too much stress. Starting out with 10-15 mins a day, and working up from there. Corns have tiny teeth, very thin gloves will suffice. Still might startle the heck out of you though
When I've had people tell me that they feed out of the enclosure in a separate feeding tub, I've only noticed it to be for one or both of two reasons: avoiding substrate ingestion and possible aggression issues. Both can be negated in my opinion with the right approach.
Ingestion of substrate will always be possible when feeding in a viv, but a snake would have to ingest a decent amount and regularly do so to be an issue for impaction. A tiny piece by itself won't cause harm. Obviously they do so in the wild (although if they did die from it, we wouldn't know right?) but it would cause so so many deaths that it just doesn't happen a lot. More often than not, their rather robust digestive system would just pass it through or break down what it could depending on the material. My snakes don't love it but they allow me to pull off any pieces stuck to the mouse with tongs while they're working on it.
None of my snakes have mistaken me for food, but the kingsnake I have is sometimes INCREDIBLY eager to come out and inspect me, in these situations i make a fist as i approach him. Very obvious that it's too large to be food, and after he's inspected me, I'll continue on to do whatever else. Never been bitten by him except the one time i dropped a mouse and went to pick it up with my hand, like an idiot.
Bit of a ramble there I know...
