I know that a hollow black hole (a sphere) will likely collapse into a singular point.
Not "likely", but rather "certainly", and very quickly too. The proper time it would take for the shell to reach the singularity is a fraction of a second for a stellar mass black hole.
But what would happen if an observer either inside or outside of the sphere crossed the surface of the shell?
There's no way for someone "inside" the shell to go backwards; once inside the event horizon, everything must inevitably fall towards the singularity. In effect, the formerly spacelike direction inwards becomes timelike, and the singularity is in the future (and just as unavoidable as the future is for us). For someone "inside" the shell, the shell is in the past, and trying to go back to it is as futile as trying to go back to yesterday.
And how would it bend spacetime? I'm assuming that the inside would experience no time bending and the outside would be very typical of a regular blackhole where the whole mass of the sphere and its contents are used to calculate the bending of it
I would not assume that. Black holes are the epitome of objects that require the full relativistic treatment, so any reasoning about them based on Newtonian gravity is extremely suspect indeed. Certainly from outside the event horizon there's no way to detect any internal structure of a black hole, other than its mass, angular momentum, and charge. This is the famous "no hair" theorem.
