And the US can't be trusted, surely Russia knows that by now, and wouldn't make a secret deal with the US, since it normally doesn't keep its end of the bargain. Remember what happened with the NATO expansion promise, and Yalta which was a legit agreement, then suddenly the West backed and started laying claim to areas outside its' agreed area. I don't think Russia would bother making a deal about it, and I think this was relatively unexpected for Russia too.
It might be that there was a scenario prepared and a script to follow for Crimea. Frankly it worked a little "too" well.... but that Russia had not anticipated quite the turn of events that expired in the Donbass. So for Donbass Russia had to improvise. Hence the long silence while Russia was figuring out how to handle it. My impression is that most of what happened in Donbass was relatively spontaneous, and Russia chose to mildly encourage it, until suddenly it appeared to make a u-turn and pretend it was not involved.