Anytime, but I generally add mine closer to flame out to help "save" room in the kettle since sometimes I'm pretty near the top and a pound of anything takes up a bit of room.
No. Unless you are doing a small boil, I guess, so that the wort would condense and create maillard reactions but that would be a stretch as there isn't that much sugar in a pound of lactose as far as I know. I think it's (9?) points in 5 gallons. So pretty negligible really.