1) What are the chances that this miner can be repaired?
90% if a Profesional with the right tools and spare parts fixes it, slim to nothing if a regular person like myself does.
2) Can it theoretically run with less than all 3 hashboards (like my S9J does)?
It can run with any number of working hashboards.
3) Am I correct when I say this is the most power hungry miner that can (only just) be plugged into a standard 240V 10A power outlet?
not the most power-hungry, some miners consume up to 6000w, the second part however is somehow correct, this needs 220-240v, but 15A not 10A.
4) Does the info on the PSU label ( specifically OUTPUT 1: 14.5-21V 170A ) mean that this PSU is NOT a 12v DC PSU and that I will not be able to use if for other miners (in case the hashboards are toast)?
The PSU can't be used on other miners, not even other miners from the 17 series itself.
5) What questions should I ask to clarify if it is worth considering?
Can you sell it for $20? this is the only question I would be asking.