Don't do it if the bull's way too heavy for the heifer and you can't get semen from him sufficiently to AI her. Most herd bulls are mature beasts, but if he's a yearling or a small bull, then go...
No. The bull should be separate from the heifer and her calf simply because it's less stress for her and enables her to mother up to her calf without having to be getting after the bull if he tries...
No. Heifers are female, bulls are male. Heifers cannot change their sex like some other creatures can. However, heifers that were born with a twin brother and shared the same placenta with her twin...