"If women had genius IQs"
Failure in the first clause. Women are individuals. Some women are stupid. Some women are indeed genii. You cannot meaningfully generalise one half of Humanity.
"what would they do with that potential?"
See all the comments. For a few more examples, look up Mary Leaky, Meave Leaky, Jane Goodale and Diane Fossey. And that's just a few especially famous examples from one area within science.
"But there's not a lot of motivation for nature to grant woman with great intelligence when she only needs to fend off danger from her young, and leave all the heavy lifting to the man."
Of course, this has never been the case. In many societies, women were (at least theoretically) in charge of the domestic domain (no, not just a glorified maid, actually in charge), and then there are, of course, the wives of nobles and merchants who had to lead at home while the husband was abroad, with his time of his return uncertain - or indeed, that he would return at all, which brings us to another point: Widows. One cannot leave the heavy lifting to the man if said man is dead. Indeed, one less obvious aspect of the romantic archetype of the wealthy widow is that managing to remain rich demonstrates her to be an intelligent and prudent woman.
Not allowing women to be intelligent is a luxury only families with easily managed wealth can afford. And as this became more widespread in the 19th Century with the growth of the upper classes, it turned out that the female mind is not actually equipped for just sitting around all day looking pretty (in a virteous way, of course) and having children, leading to "hysteria" (depression and neuroses due to extreme chronic boredom) and feminism.
"A woman could have high test and this testosterone-induced STEM type genius, but she would also reach a point of looking too masculine for her own good."
I am a very intelligent male, yet I would not call myself "masculine" in appearence or personality (not that I am in any way "feminine" or that I actually believe in either). Intelligence comes in all forms and shapes.
Further, not every man has the same taste when it comes to the ideal female form. One man's ugly mannish hambeast might be another man's wonderful strong Amazon (and remember, pregnancy and childbirth are not exactly easy on the body even with our modern medicine. In the old times, it was far, far worse.). And while physical attraction makes far easier to notice the "Certain Something", I would always choose the woman who I would consider physically unattractive whose great mind thinks alike to mine over the one with the beautiful body, but nothing on the inside.
"Perhaps this is why they say when it comes to heritability of IQ, the woman's IQ is not as significant as the man's IQ in relation to the potential IQ of their offspring."
No, it is simply that many mutations that affect intelligence are located on the X chromosome. If a man has a recessive mutation on the X chromosome, he is affected, while in a female, it does not affect her unless she is homozygous (actually more complicated than that since only one X chromosome is active in every cell, and the distribution is pretty much random).
"She carries the potential for genius in her genes, but for a female it's not that necessary to have abstract supercomputer levels of problem-solving skills when you're oriented toward more pragmatic needs as opposed to sitting in a dark corner pondering astronomical, borderline schizophrenic things."
But I thought women don't need to be smart because the man does all the every-day stuff...