You got the first piece right.
When I say "using either method may not matter if you abandon the level with no progress," this is because you don't get any AP if you leave a level right as you start it. Your starting territory doesn't count towards earning AP, Fizzer can prove this or correct it if I'm wrong.
As for knowing how much AP you'll get for partial completion, if you exit the current level your in and try to start a different level, without using a multi-level item, you get a pop-up box where you can confirm if you want to abandon your current level. In this box, you'll see the following:
*number of territories you claimed/total number of territories in the level
*Current AP pool (Not including what you have already claimed, if any)
*AP you'll claim if you leave now
When you see the box, it will look like this (these are for my current level):Starting this level will abort the level you have in progress. You will earn partial AP for your progress in this level. Are you sure you want to do this?
*Territories captured:212 / 575 (36.87%)
*AP earned from victory:208.28
*AP you can earn now:31.98
If you want to know how AP you've already claimed from a partially completed level, it will say how much next to how much the level is worth on the level select screen, in parentheses.
Using my current state as an example, this is what you would see:
*Worth 233.76 AP (25.48 already redeemed)
Hope this helps clarify some things.
Edited 10/7/2020 07:15:20