Just a quick question: If I normally use my zone 2-3 starting at a
station that does not have PAYG (say Harringay) to travel to a zone 1
station (say Moorgate) while I have a zone 2-3 travelcard on my
Oyster, would the system assume I touched in within my zones and
charge me the correct extension price, or would this be an unresolved
journey?

How about the other way around, i.e. starting in zone 1 without
touching out with a zone 2-3 Oyster card.