The situation is complicated but it boils down to the rebel groups making a decision on what their priorities are, and dealing with the consequences of those decisions.
They face having to choose which is a greater threat. On the one hand, going against the Nusra Front can result in their extermination. On the other, siding with the Nusra Front, in an attempt to fight the government, renders them targets.
In terms of the U.S. and Russia, it is encouraging to see that despite tensions, the nations are willingly to cooperate with one another when facing a dire situation. The nations have had a rocky history, but they tend to put their differences ( however large they may be) aside for the good of humanity. There is historical proof from the time of the Cold War.