1. Seiza gender differences
The prescribed gender differences in seiza one sometimes hears stem from traditional Japanese seating etiquette, analogous to sitting sidesaddle/astride in equestrianism, or with legs crossed/spread on a chair.
These differences are echoed in texts on other traditional Japanese practices which involve seiza e.g. buddhism, chado, shodo, ikebana, kabuki, kyudo, aikido, kendo, karate and appears to have been the norm in the Kodokan from its earliest days to the present:
However, with the rise of feminism in the 20th century, discriminatory practices in judo have been abandoned in many societies (e.g. gender-segregated training, striped belts for women etc), and as such nowadays few dojos outside of Japan expect women to sit differently from men in seiza during judo sessions.
This was not always the case, something which many people took umbridge with in the past:
These are the male sensei who seem to have the knack of teaching women, especially those women who have strong feminist consciousnesses--the women who resent striped belts and sitting with their knees modestly pressed together in seiza.
- Black Belt "What do these women want?" (Apr. 1975) (p.22)
In Joshi or Woman’s Judo, persons who hold a blackbelt rank are awarded a special black belt that has a white stripe down the center. This belt is only awarded to a woman. And represents the Pure form of Judo as they do not rely on strength like most men do. However most woman prefer a normal black belt like men.
Note that in Japan there is also traditionally a proscription against women sitting cross-legged (agura):
The Seiza (formal sitting) should be the rule of the Dojo. Women are not allowed to sit cross-legged at any time. Even when resting during the session, you must maintain good deportment.
2. Hand placement in seiza
Traditionally in seiza, there is no gender difference in the placement of the hands on the thighs:
[The bows are performed] in the same way, in both men and women, except for the difference in the opening of the knees...
As for where exactly to place them, long as your hands are resting on your thighs there should be no qualms. Different texts sometimes state different specific positions:
The upper body should be straight, the shoulders not square but in a natural manner and the hands placed on mid-thighs. This is called the Seiza.
Rest your hands lightly on your thighs at the place where they join your hips.
But many don't specify at all:
3. Hand placement in zarei
Traditionally in zarei there is a slight difference in the position of hands between men and women when performing the more casual version of the bow (senrei), but it is generally something more akin to futsūrei which is performed in judo, with the hands forming a triangle in front of the knees:
And, in the case of a woman, align both hands at the kneecaps and attach it to the floor as shown below.
In the case of men, the position at which they put their hands when bent is different from that of women.
Men stand parallel to each other in front of their knees, keeping their hands apart.
The other points are the same as women.