I obviously did this wrong, so please tell me exactly how you can exit from the cosmic circle to be able to continue playing the game after completion.

I made sure I had literally tons of projectiles for slings & bows since I've read you can't pick anything up.

I defeated the savant and then started running for the exit immediately. I got out, got back to the top of AP, but the dialogue continued regardless and eventually the game completed.

Do you have to get off AP?

I think it depends what option you choose just before the final battle and which side you ally with. I've never tried it myself but I'm sure it's been discussed on these boards before.

xfactor

I've always managed to do it by picking the "tear out a page" ending, and then killing first Bela, and then the Savant. Afterwards, the game controls lock up a bit, but you can still move, so leave the Circle and wander the Peak until you find some monsters to fight. Once battle is initiated, the game controls will return to normal and you can continue the game as normal, and even return to the circle to pick up the Savant's glove.

What he said. Check the hints&tips sticky, I posted something there awhile back on the neverending game option.