I'm looking for answers to questions to the following. Should the head be as close as it can be to the crotch?
It depends on the anatomy of the legs. If she is a bodybuilder and her thighs are massive, they may rest on the jaw and head, rather than targeting the neck. In that case, she may have to use the area around her quads which will have a smaller circumference. Slimmer, athletic girls can crank up a front-headscissor, with the chokee's throat in their crotch, whilst using their hands to pull on the back of the head. This causes a blood and air choke combination, which is more painful and distressing, as it occludes the trachea.
Should she be trying to squeeze with her thighs or flexing her thighs?
Impossible to squeeze without flexing the muscles, surely?