months near the blackhole, minutes away from it.
In the lifts the time dilation would ease as you went away from the blackhole so (making up numbers) if it took 6 months in top-time for the proto-cybermen to go get Bill and come back, it took the proto-cyberman 3 months, Bill 1.5months and the Doctor/Missy/Nardole minutes.
I think anyway the time dilation for someone moving back and forth between the two spaces would be half of what the slower time line experiences.
That all works I think, but there must be some reason why the other people don't just get the lifts to escape their horrible environment. There is some dialogue between the Master and Bill about Bill not being told where the lifts are or how to get back up so perhaps the master has kept this knowledge to himself. I bet it won't get explained but I think this is the the best explanation, that the master held all the original people and the knowledge was lost as the generations passed, except to the master. Or simple physical control over the space.