all it would take is some lore babble about cawl messing around and creating a few female space marines, then introduce a female head sculpt for space marines and she would play space marines.

So completely re-write existing lore...not exactly a small ask

For eldar she struggles to find any models

I suggest she looks harder then. Howling banshees would like to say hi.

It would go a long way to update the box art to show any female head options.

Errrr the new guardians quite clearly have female sculpts in them.

How about instead of coming into a hobby and feeling all entitled to change it. You come into the hobby, respect what has happened and find a faction you like. The cheek that people expect to be able to re-write the lore to fit their narrative is astounding.