It is possible to enter a different delivery address when you're completing the purchase in the resale. I helped folk out for 2019 and managed to do that, worried the payment would get bounced if the addresses didn't match the one on my bank details.
It won't lock anything at that stage. If you miss-type reg or postcode like that, it will fail the registration database lookup and give you an error on the reg number input page. The lock is only placed on the reg numbers once you move forward from that point to give you time to complete the purchase and to prevent contention occurring if two separate sessions (two people in your group) are trying to buy tickets at the same time.