United - P+ empty but no economy seats available for sale

Four of us (2 adults and 2 kids) will be traveling MEX-EWR soon on award ticket in economy. When I looked at the seating chart, just about all the "free" economy seat selection are taken. A few seats in the "preferred" and "plus" economy are taken.

Then, out of curiosity, I took a look at the flight for tomorrow morning. So, I assume most people would have check in at this point. For tomorrow morning, UA is selling a ticket for business class cash, but no economy seat of any kind. Yet, 27 of the economy plus seats are not assigned. All economy preferred and regular economy (the free economy) seats are taken. So, it got me thinking the following:
- if I were traveling on basic economy, then, won't I get an economy plus seat at boarding? This assumes no seat assigned yet.
- If that were the case, won't traveling in basic economy be better in this particular case?
- Would United be selling the economy plus seat at a big discount as boarding time gets closer?

Thank you.