(I’m not feeling very well, so I don’t want to go into a lot of technical detail, but I’ll give you a quick answer and others can fill stuff in if they think I’ve missed something.)
Assuming the sensor is eligible to run locally…
The short answer is that the sensor will figure it out. Zigbee includes signal strength in the process of deciding which repeater to try. So while the switch is off power, if the hub can still be reached and is in the sensor’s neighbor table, the hub will be used. Once the switch comes back online if it has stronger signal strength than the hub, then it will probably be used again. This assumes both were in the neighbor table to begin with and that the switch didn’t leave the network, it just went off power.
If after power comes back on you feel that things didn’t connect up again the way you wanted, you can just do a network heal at that point. It can’t hurt anything. But it shouldn’t be required.
As far as network analysis tools, at the present SmartThings gives us almost none. They have said there will be more available in the future, but no specific timeline.
Meanwhile, there are some third-party tools you can use if you want to do so.
As far as zwave, you would need to add a secondary controller to your network, probably an inexpensive Aeon stick, and then use mapping tools with it. I know some community members have done this, IIRC @Fuzzyligic has done so, perhaps he or some other community members with experience in this area can comment further.