There is also the possibility that Russia might try to cover up all the signs of a chem attack. Before the rebel surrender, Russia said that there is no need for any UN inspectors, and had blocked a UN team from getting to Douma. After the surrender and the moving of rebels out of Douma, they have invited inspectors in the site, which means that they probably have done some tampering with it.
I think that who did it is not really a question, as Russia still denies the happening of a chem attack, and if some other party committed it, then it would very likely show up evidence from Radars, cameras, etc, of who was in the area when the attack happened, and to try to implicate them in it. It would prove of a very high value to Russia if somebody else did it, in terms of moral support, PR, and other things.