This issue has been resolved with help from an Airbnb staff. They suggested to
1) remove the "arrive within x months" criteria thus opening all times on the calendar
2) select the first date of unavailability
3) in the right-hand info box, the end date of the selection can be manually selected till the 'end of time' i.e. April 2019 as of this stage.
4) Presto! All dates are now blocked until then and there is no automatic window to deal with.
@Clare0: Unfortunately, this doesn't solve your problem completely as you yourself have a moving window of availability, unlike me. However, with this solution you only have to do half the work i.e. make the 4-month window dates available, and not make the 6-month ones blocked.