BBC News have just interviewed another "expert" (worked on the 777, now runs a Flight Safety company). Some of what he said lines up more with where I've got to on this if you give the crew the benefit of some doubt:
- as they coast out ACARS fails for whatever reason but unknown to crew
- catastrophic event at IGARI, takes out most other comms / electronics including transponders
- perhaps an explosive decompression
- maybe flying controls degraded too so are using differential power perhaps explaining altitude changes and jinks in heading, either way they now have their hands full
- aircraft barely flyable so they try their best to get back to base, explains left turn if primary radar signal really is MH370
- then in trying to fly / navigate and with no comms it just all gets too much (with / without oxygen) and they're left heading out to sea on whatever heading and altitude they'd managed to get it to but their efforts are all in vain and it goes down, somewhere deep, west of Malaysia.
Something like this still seems plausible to me, more than suicide / heists / bullion / terrorism.