This isn't meant to be complete, more a collection of points addressing some of your sub-questions.
In other (engineering) fields I've used practical tests as part of an interview process. If you're interviewing, that might be an option - but remember that people won't necessarily be in workshop clothes at an interview.
In some fields in some areas, trial shifts are common - paid work but with no commitment on either side.
If it's a community shop, are all your roles paid? One community enterprise I know of has a program for providing bikes to those in extreme need (e.g. refugees), and that's mainly staffed by volunteers. Some of those volunteers go on to pick up some paid work. (I may be a little hazy on the details, that's what I've pieced together including from eavesdropping). You'd need to be careful around labour laws.
If you have sales/shelf-stacking roles, you can take people on mainly for that and train them as well, starting with the easy stuff. It doesn't have to be very formal to start with, but sooner or later you might be looking at qualifications which will likely incur external costs.
You should look into what support (if any) is available in your area for taking on apprentices. This is a bit of a commitment but can work out very well for the business and the individual.
As for whether bike mechanics are born or made, it's obviously going to be harder to learn to work on sometimes tricky mechanical systems if you lack practical skills of any kind. But someone with skills in one area can easily learn in another area. For example I learnt to maintain my own bikes in my 30s, learnt to build wheels in my 40s while injured from a bike crash (still with notes in front of me) - but I've been aligning laser systems in work for 20+ years. The skills are surprisingly similar even if the levels of cleanliness are very different.