Is it permissible to annul the marriage contract and divorce the wife, or to marry another while remaining married to the first, knowing that there is affection from the wife and her family's awareness, and the ability to support both wives in case of polygyny; and must the full deferred dower be paid, or half of it, if a divorce occurs?

We do not find in your question anything that necessitates divorcing this woman and separating from her. You should be a support for her in completing what she lacks. If you are capable of providing for, cohabiting with, and being just towards a second wife, then there is no harm upon you in marrying one. Regarding the law that prohibits polygamy, a valid, religiously sanctioned marriage contract can be concluded outside the framework of official institutions, such as at an Islamic center or the like. If you insist on divorce, then you are obliged to pay the full Mahr.