G-HUET as well as all three ATR72s went to Malaga last winter and onward to Las Palmas for maintenance. One of the legs from GCI to AGP and return took pax - a special put on by Aurigny.
As for the Dornier saga, one airframe can do the required flights over the winter months, but the schedule requires two - one based overnight in ACI, the other at GCI. Normally the Guernsey a/c goes to ACI and on to SOU, while the Alderney aircraft begins the day with a trip to GCI. If flight times are adjusted for one a/c to do the lot, starting from ACI, it means a delay for the GCI-ACI first flight of the day and a delay for the morning ACI-SOU sector.
Forgetting G-SAYE (which is a bit of a disaster with its limitations and no longer flies) there should be one spare airframe, not two.