A pre-approval should NOT lock out the dates. According to AirBnB, you can pre-approve multiple requests and then it's first-come/first-served. (Basically, a pre-approval is like a one-time "Instant Book" permission). In addition, pre-approvals expire after 24 hours, so whatever "semi-hold" they might SEEM to put in place will soon go away. Lastly, saying it another way, even when there's a pre-approval in place, if anyone else requests a pre-approval or a full booking, you should be able to approve that.