With the reason why girls make better magic users than boys, I've always headcanon'd it more on the female puberty/hormones part of it. As we know, hormones and puberty tend to screw with peoples emotions, could be that it just so happens that girls in early puberty have their minds naturally altered by said hormones such that they are more responsive to the method of unlocking magic the incubators use, with the creation of magical boys, adults and/or children being more difficult, either impossible or inefficient energy wise.
As has been pointed out, it's not like teenage girls really have stronger emotions than boys or adult women, those emotions just express more openly which could aid in some conceptual catalysing connection necessary to the magic unlocking process, compared to other demographics with more closed off/stable emotional states, which is the reason for this ease of magic unlocking.
Could also be something to do with the fact puberty is a time of great change in a humans life, and this change can be conceptually harnessed in the transformation/creation of a soul gem, though this would also apply to boys. Maybe something to do with the way boys tend to close off more meaning extracting the souls is more difficult?
Magic strength I'm also absolutely certain does not correspond to strength of emotions, as Madoka's magic strength increased due to timeloops which shouldn't have affected her mind personally at all, more evidence that the emotional state is either purely a catalyst or only part of the equation.
Personally, I believe it is fate and the future effect you could have on the world which dictates magical strength, and your emotional state which dictates if you can be unlocked, with only those who combine those two factors at a point in time having 'potential', which is why Kyubey just drugging someone with hormones into the optimal state for contracting doesn't work well; they still need high fate potential which Kyubey can't scan ahead of time, and likewise even if you have high fate/magic potential that isn't helpful if you don't have the specific mindset brought about by female teenage hormones which allow Incubator unlocking to work in the first place.
I er, I didn't mean to put this much effort into analysing magical girl creation mechanics. Oh well.
You've got to admit, "I'm an oversoul projected in a form of a forum quest from the world of a higher ontological order consisting of several dozens incomprehensible and possibly insane beings and also a daughter of two goddesses that haven't come into existence yet in this timeline who experiences time in a non-linear fashion" is a tough pill to swallow.
Heck, we didn't tell this stuff to Mami, they're just people on the Internet we've met.
That they're just people on the internet means they would be great candidates for sounding the theory out
Also neat plan.
[X] Raiseth