Actually, you can put on Danielle's "clothes" via the tailor mod, so yes you can do it in the build.
The starting char choice (Blaze or Danielle) was disabled because it changed the character's _ID_ as well as model, thus breaking various quests.
As long as you only change _model_ (as above), you'll be fine. Well, that and animation and sex too.
I highly recommend doing it with the Tailor mod, since that automates `model-changing`, you just pick what clothes you want and the game handles the rest.
Regarding standing: Yes, that's the other main reason why using a female model is difficult (for pchar or officers), the "woman" animation lacks the sit frames.
However _usually_ what happens is the model _disappears_ when it's forced to sit; consider yourself lucky that you're at least still visible. <img src="http://www.piratesahoy.com/forum/style_emoticons/<#EMO_DIR#>/icon_mrgreen1.gif" style="vertical-align:middle" emoid="

" border="0" alt="icon_mrgreen1.gif" />