Should questions regarding the care and repair of our instruments be considered on topic here? Why? Why not?
Could that be extended to questions about making musical instruments?
Care and repair should definitely be on-topic. It's closely related to Musical Performance and drawing a line between them would not be useful.
I'm not as sure about instrument construction. While there's certainly a large overlap, I would assume the majority of relevant questions are irrelevant to the average musician. That said, I would have no problem with this site being host to them unless the Musical Instrument Construction, Maintenance, and Repair proposal takes off. If it does we would presumably migrate our own care & repair questions there as well.