Walter
Your alternate track is fascinating and, like the CPLS theory, certainly feasible. However, the RUC interviewed 8 witness who placed a Chinook on your Aldergrove > Waypoint A track at the correct times; from locations in Antrim, Moorfields and Carnlough/Glenarm.
Of the other 4 witnesses, one merely reported "a helicopter" up in Dervock (roughly on your Waypoint H > B track). Another saw a Chinook over Ballymena, but this was more than 2 hours earlier.
The only real discrepancy comes with the final two witness, one ex-Army and familiar with Chinooks, who (separately) saw one heading from Ballymena to Carnlough (roughly NE) at 1730. My memory of the topography is very vague indeed, but I'd say it is possible they are describing an event whereby the aircraft took a westerly detour at low level around Slemish Mt to rejoin the main track further up the Carnlough Road.
On balance, and lacking data recorders (!) I'd say it is a fair assumption they were on the direct Aldergrove > Mull track, with perhaps a slight detour around hills.
Sorry for any terminolgy slips. I look at these things strictly from sea level.