I entered Dublin-BKK and selected upgrade both legs and WTP as the class of service. There was availability for CW in both directions and it costed only about 1500 Euro + 39k Avios.

That's less than a WTP booking would cost so I can only assume it's doing it by booking into WT and paying the Avios to upgrade from WT to CW? I assume I'll get the miles and TP for WT though I have no way to to determine whether I'll get 35+10 or 70+20 TP?

How does it come out to 39k Avios for a symmetric booking? Is the upgrade cost really 19.5k Avios? That seems pretty random.

If I choose to upgrade only one direction the price jumps to 2000 Euro for WTP in one direction and CW in the other. That seems to justify my assumption that it was booking into WT for the upgraded leg. The number of Avios drops to 19k which seems to imply for some reason the number of Avios required is 19k Eastbound but 20k Westbound? That seems strange.

How can I get it to book into WTP and upgrade from there. That would ensure I get enough TP *and* save Avios. Also, is it costing extra to upgrade the LHR-DUB leg?