a) With only 8 first class seats on the a319, it is rather unlikely that a seat will open up. but...
b) if it does, you will be among the first in line for it.

HOWEVER, do note that on redeye flights first class service is cut way back. You may or may not get a snack and breakfast will be juice, coffee and a danish.

Further, as you may already know, UA's domestic fist class seats are not that much more comfortable then coach seats for sleeping. They are narrow and do not recline very far. While the bending headrests help, its not all that much.

Having said all that, however, I do tend to upgrade redeyes myself....

Welcome to FT! This question reminds me so much of the very early days of FT !!!!!