the reason why Gideon did what he did was because of his obsession to the fake bish. he probably didn't care much abt losing his powers once the regression happens because as long as the bish will be with the crown prince, he (Gideon) can get her.
this gets me to another theory that whatever info was in that missing page, it's all about the soul switching after the regression. so Gideon (now in the CP's body) lost his magic as per the reqs as an invocator, can get the fake bish and do whatever the fck he wants with her.
reading ch. 3 without context is giving me a headache. WHAT DO YOU MEAN "MANY LOVERS?!?!"
Well, Shirou is known to be a playboy he has many partners... but Renge considers it Shirou's lovers so yeah...hope this helps ease your headache ^^
and Renge still considers himself as "one of many" despite the fact. no it did not ease my headache at all TT