I tried to recalculate the northern route as well using lower speeds (400-420 kts).
At all speeds, it slices off a bit of Eastern India, goes above Himalayas between Nepal and China, and terminates in Tajikistan or Kyrgyzstan.
Below 420 kts it may never even get out of the mountains: there's a pretty formidable and desolate mountain range with peaks up to 23000' across the route along Tajikistan/Kyrgyzstan border, right in the area where the "partial ping" could have come from at 400 kts.
I know, I know, "how come no one saw it on the radars?" Suppose that Indians missed it for whatever reason. Nepal has no radars and even no air force. The rest of the way is over pretty rugged terrain, mostly above 15000'. My routes even pass in the vicinity of K2. We already heard that the aircraft flew lower than normal (FL295) in the Straits of Malacca. Flying FL295 in the Himalayas could significantly limit its exposure to radars.
The biggest thing I don't like about this is that slower routes take it into the southwestern corner of the Tibetan Plateau. It's still pretty desolate, but it's flat and the aircraft could be visible to Chinese military radars if there are any in the vicinity.
P.S. If you think that Tajiks would have reported a big aircraft crashing into one of their mountain ranges or at least would have gone to look for it, then a) the area we're talking about looks roughly like so
http://www.panoramio.com/photo/49758018, and b) Tajiks have bigger problems than looking for other people's lost aircraft:
http://en.itar-tass.com/world/725561