Kids, eh?

1) & 2) Well, sometimes dominance can appear under circumstances like this (death of former Boss cat; emerging adulthood), but sometimes sex is all in the mind (i.e., removal of gonads is no guarantee).

3) Unfortunately, sexual behaviors can also be triggered by an ovarian remnant. Very occasionally, surgery to remove a female's ovaries can leave a wee, itsy snippet behind. We know now that these remnant tissues can activate and produce estrogen. Females with active remnants can exhibit all the signs of cyclic estrus ('heat'), and males can find them attractive. When all other causes are ruled out, a blood test for estrogen may reveal if she needs a second surgery.

1) & 2) You're using a spot-on treatment at the shoulder blades? You may have to separate them until the oil spot is no longer visible on the fur. Try a feline happy hormone diffuser to address behavioral imbalance due to stress/emerging dominance/bored-sexy-teenage-whatnot. Make sure he is exercising and having enough mental stimulation in the environment.