I was thinking of a hinge in the extending arm at the top end of the drone (so opposite end to what you have drawn), and make it spring loaded. When everything is packed in the tube, the arm will be straight, but once the arm is extended out of the tube, the spring will bend the arm up 90° or so. That configuration should be pretty easy to draw the arm back into the tube so the whole apparatus can be removed from the borehole (just need to overcome the spring when you retract the arm). It would have the disadvantage of the spring being the only thing holding the drone up, so you'd need a decently strong spring.
I'm not sure how you'd get the drone back out, but in a quick search of companies that do mine drone inspections, all the drones seem to have cages around them, so I'm guessing crashing is pretty common and therefore your drone might be disposable anyways.