Pinkman
On the balance of probability, the "I can't believe they did this" scenario gets my vote. Albeit with a caveat, that being that I actually can believe they did this. It is commercial aviation in 2016.
If no one in the cabin can tell the difference between Jet A and Illy coffee and the third pilot was too lazy to go back and have a whiff then SIA has some real problems.
And what was the EICAS telling them?
Immediate diversion to BKK will likely turn out to have been the correct response from a safety point of view but as we all know commercial is in charge.