Your very first unedited post says:
Originally Posted by
FlyinPete
With the recent attractive J fares I booked a couple of tickets between London and Tokyo, on the MH website, with separate tickets dovetailing into each other.
So you might see why everyone assumed the separate ticket was the KUL-BKK portion.
I've had similar things happen with reschedules. I found Cathay Pacific very easy to work with via email. They nearly bent over backwards to get us on a better timed flight even though it was a BA issued Avios award ticket. BA themselves wasn't willing to do anything unless award space existed on an alternate flight, but CX had no problem opening up inventory for us directly. Another issue with Air Canada took multiple calls but finally Expedia accidentally put me through to someone at AC (they weren't supposed to and the AC rep wasn't happy) but finally I got straight answers and the flight reschedule issue resolved.
With patience and a humble tone eventually, usually you can get accomplished what you need. As you've noted, KL and Fiji aren't very useful so the poster's recommendation of speaking to someone at LHR directly sounds pretty worthwhile.