Guess so, but most of these bikes have not been ridden much so I suspect the chain and cassette have got 1000s of km to go. They're 9sp and the economics of swapping out the chain at (say) 2500km is marginal: as opposed to letting them grow old/worn gracefully together.
For
@Sharky Have you had success with a flat bar friction lever operating a 9sp cassette?
For the OP: the shifter I've referenced is a brake/shifter combo: I don't know what 7sp aberration someone has fitted on the bike you've bought.
For the OP: Your Option 3 will cause nause as the shifting will be poor: I would be embarassed to offer my son a bike where the gears don't work properly. Sure fire way for him/them not to ride the bike.
