We are able to add points upon level up (I do this in my Lone Wolf mod), but I don't think taking away points is possible.
Skills should be able to have stat requirements for them. I know at least one skill has a specific intelligence requirement in it's txt file. I don't know about whether multiple requirements would be doable though.
The effect of the stats upon the skills themselves is hardcoded into the game. Any spell within the five magic schools will lose their Int cooldown reduction bonus if they move to the non-magic schools.
I'm not sure about weapons. I think it might be possible to require multiple skills, since if I remember correctly their stat requirements are listed in the item stats.
