Well, considering that none of us can know what happened for sure, and any claims by any side could be attributed to mere propaganda, all we can do is hypothesize and estimate probabilities... I think there are following possibilities out there
A-priori all these scenarios were equally likely. As time goes on, we should expect to come to the only most likely case.
You are claiming it was definitely 1.a.i., USA officials do not blame Russia directly, so it is 1.a.ii according to them
Russian and Syrian official claim it was either 3 or 2.a
So given those claims 1.a, 2.a or 3. would probably have 90+% probability with others to be rather negligible.
Now considering that it was not in interests of Assad or Russia to provoke any additional international critisism, I would say 1 has less than 20% chance, I would put most of my money on 2.b or 2.c.
"Only the hypocrite is really rotten to the core." -- Hannah Arendt.