If we assume "etting Snowden fall into the clutches of Russian intelligence may be a far better deterrent to future leakers than a 'clean' prosecution and imprisonment here in the US", then surely if the Russians are capable of thinking long-term then treating Snowden nicely is in their best interest (why would they want to play into discouraging future American leaks?).

Depends on their belief in how my they've learned from him that we haven't independently figured out, normally you don't do that to make the counterintelligence game harder, e.g. does the US know all the methods he used to get his data? Plus if they want any future "defectors", as we called them in the bad old days, to come to them they've got to appear to treat him well.

Ship him back? He knows a lot about secure IT systems in the USA. If they are smart, they will keep him around to advise their own signals intelligence / hacker people on how to penetrate US systems. Maybe feed him some propaganda, help to push him a bit further over the edge (this is a guy who was at one time a big believer in the NSA, after all).

Why would they ship him back? That would be just a weakness. They can afford to keep him.

On the other hand, why wouldn't they hang onto him for a month or so, see if they can't extract information from him (not even talking about torture, there are much better ways of getting information from people), then ship him back to the USA or send him on his way someplace else. Long-term, he's served his purpose, no reason to spend a lot of money on him, especially if they want to smooth things over with the USA. And that's basically exactly what they're doing now: saying he can stick around until someone else claims him. Whether that's the USA convincing Russia to hand him over or some nation offering him asylum, Russia probably doesn't really care.