Yes, you'll still be able to access the house. In fact, the house becomes even more useful LOL. You can teleport to it, use its merchants/vendors/chests, and then exit through the door, which takes you back exactly where you were before.

Regarding your later question: you receive a warning that it’s locked content, and you can’t teleport there. Only the internal teleportation is allowed.

The exterior entrance may be in a zone, but the house itself, any of them, is a separate "zone". Instanced homes are the only way to make homes unique to the account. It works much like the solo instances of the Harborage, where each player goes into a separate copy.