So far as assimilated drones being able to be de-assimilated and then being able to adapt sufficiently to return to their former lives, I think it depends a lot on how old they were when assimilated and how long they were drones. Seven of Nine was a young child when assimilated and had been raised by the Borg, spending the majority of life as a Borg. This is why she had so much trouble adapting when her link to the Collective was severed, and why her reaction to her returning memories during her previous separation from the link was so different from the other drones in her work group in Survival Instinct.
Compare it to people who are released from prison after spending much or most of their lives in prison. They often have a very hard time adjusting to life in the outside world again: having to make decisions for themselves again, the normal responsibilities of getting and keeping jobs, mundane necessary activities of everyday life, etc. In prison, all decisions were made for them and they didn't have to think about or attend to these things. So, many of these people end up back in prison, because it's familiar to them and they know what to do to fit in there.
So it would be for long time drones who are severed from the Collective. Those who do well are those who are fortunate enough to be put into a structured environment with set routines, and are given specific tasks which make them useful. This is no doubt why one of the very first requests Seven of Nine makes is to be given work to do.
Those several ex-drones who are left to shift for themselves in ambiguous, unstructured environments with no routines and nothing useful to do are the most likely to return to the Borg on their own and voluntarily seek re-assimilation.
And, in reference to comments above about the nanites being the Borg, I think its probably impossible to remove them 100% and where they can't reactivate and reproduce at some point in the future. For those ex-drones who do resume their lives as individuals, like Picard and Seven of Nine, the nanites remain, but lay dormant, until proper conditions reactivate them to greater or lesser degrees, depending on how long the person was a drone and how soon after de-assimilated it is. This would explain the difference in their reactions to relevant stimuli it was for Picard and Seven of Nine in Raven. Picard merely hears whispers from the Collective, where Seven leaves the ship to try to return to the Collective when merely coming within range of that homing beacon. In essence, then, drones may leave the Collective, but they'll always be Borg on some level, to a greater or lesser degree and however dormant it may be.