@SK999
I don't know who you are and you only have four posts here, but everything you say checks out and you just saved me the trouble of contacting Richard to see what his take on all this is. So I'll go with your number for the 18:28 ring distance: 3496km = 1887.7nm rounded to 1888 nm. For the Satellite location at that time, my location checks with Vinnie Boombatz's link: 18:28:15 at 1.5691n 64.5286e.
So there we have it, AFIK everything is reconciled. We now have a solid Longitude to plot from at 18:28, and the Latitude can't be very far from MEKAR, the southernmost Latitude possible is 6° N - the FIR boundary, (we can be virtually certain the plane didn't cross into the Indonesian FIR there.) The northernmost Latitude is more open to debate; some people here apparently believe the plane went as far north as IGOGU or IGREX, I see the path most likely being IGEBO ---> POVUS , maybe as far north as NOPEK - but that's a separate subject.
So thanks Ulric, Vinnie and SK999 along with MM43 who started this thread.