Hell. Trade them, I say. Putin's not happy with her for pleading guilty and giving up information to investigators. Let that spying bitch go back to Russia and so Putin can have her killed. What's the problem here?
The problem is she quite possibly has information that will be useful in court with the other targets (besides her and the other arrested and cooperating witnesses) of the Russian hacking investigation. The point of arresting her wasn't to take a spy off the streets, it was to be able to use the charges against her as leverage to get her cooperation in taking down some of the bigger fish in this ring. This is how conspiracy prosecutors work.
That will be unavailable to US prosecutors if she's conveniently back in Russia, traded by the boss of "US Person 1" (from her indictment papers) for a US citizen Russia purposely picked up for this exact reason. There will be no leverage to get her to help prosecute her former US contacts, and its quite likely (as you imply) she will end up even more inconveniently dead, like many other Russians involved in this affair..
Which means the next question is, if the POTUS agrees to "trade" a prisoner to a country who was otherwise perhaps going to cooperate with an investigation into his underlings, and possibly himself, is that interference in an investigation? The answer is "Yes", but its an innovative new way to do it.
The next question after that is, does anyone have the authority to refuse? I'm not sure the answer to that one. I'm thinking probably not, unless she perhaps has state charges against her too.