TG 916/917 is indeed back in the systems from 30th October (although i'm sure it will be many months until Thai release fares for October onwards..).
So i've guessed and second guessed like many on here and been found wanting as to how they could do it without penalty, all I can summise is that indeed the 'use it or lose it' rule has been negotiated with LHR. As LCY flyer has stated evening slots are under less pressure, but daily flight slots at the same time would still have a value. Indeed PR have been looking for better time slots, and GA have been applying (and been rejected) for LHR slots at most opportunities.
The last season just gone I was checking timetables, and no new flights were filling Thai's slots so it doesn't appear they are leasing the slots out. So i'm going to stop guessing now until I finally see something official. However, ultimately i'm glad as I want to see TG flying twice daily.