Is it permissible to divorce one's wife due to insubordination (nushuz), bad conduct, and infidelity, and does her right to maintenance (nafaqa) in this case fall away?

If you have provided your wife with a suitable, independent dwelling, you are not obligated to house her elsewhere. Her leaving, in that case, would be an act of rebellion (nushuz) that forfeits her right to maintenance, though her maintenance if she is pregnant is a matter of scholarly disagreement.

It appears that you have been lenient in exercising your guardianship (qawamah) over your wife. Therefore, we advise you to rectify her conduct, prevent her from mixing with non-mahram men, and from speaking or corresponding with them. Also, educate her on the husband's rights and the obligation to obey him. If she repents and amends her ways, then keep her; otherwise, it is not appropriate to keep her, as it diminishes your religious commitment, and divorce might even be obligatory at that point.