I think you are either extremely overhtinking this, or severely misinformed.

The polo has a high frequency crossover with the tweeters in the A-Pillar The crossover settings on the HU can't do anything with factory wiring as the output only has front R/L cables not separate cabling for high/low as well

Cool, so there is an analogue crossover, not a digital in the factory system that amkes this easier.

Do the crossovers go before the DC-AC convertors on the wiring diagram, or after?

wat

Do you mean amp?

There's a plug going from the glove box to the door, which I suspect splits the cabling. Is there a good way to confirm this? Resource for car looms?

Behind the head unit or at the amp is nomraly easier/better documented for grabbing speaker wires from.

You want to find the full range signal out of the factory amp or head unit, run that into your crossover, then from the crossover to the tweeter and the door speaker. You idealy want the crossover mounted in the cabin somewhere.