For what it’s worth, the TAP staff do a decent job for the BA flights they handle at Lisbon; surprisingly as my experience with TAP itself has not been great.

They police the CE/priority queue, making sure that people are in the right queue. With three desks, the middle ET desk agent often steps in to help clear the CE queue quickly, along the lines of what the OP suggests, although thus is not entirely at the expense of ET given there is a third agent. NB I’ve not seen a situation where all deans are used for CE while ET are held back and can see some problems with that approach.

I presume Lisbon gets more than two desks given passenger profile, ie a lot of connecting and club passengers.