It isn't so much that magic isn't in low supply, but the ability to learn it is. In theory, any member of the People could learn it due to the nature of the Arthrynite goddesses, but in practice, only priests and now Catclaws and some Cateyes are able to devout their time to learn it. It is also seen as something sacred due to coming from the goddesses so it is seen as inappropriate for someone who doesn't do a sacred duty to learn magic. The fact that your sacred warriors have to spend time learning magic in addition to training up to a higher standard is why you have further of them compared to regular warriors.
That is another thing that needs to be taken into account as your warriors are able to use magic to mentally shape their stone weapons in between battles. Unfortunately, due to stone magic not being a thing in real life, it is hard to tell what sort of effects this would have when it comes to determining the effectiveness of stone versus metal.
That is something they could do, at least with the stone parts. If something happens to the lead, they can't do much about that.
It is about as quick as doing it the non-magic way and sometimes slightly quicker due to the ability to get better precision and detail. Also, it isn't that stamina intensive compared to other physically demanding tasks. If you don't try to do everything at once and spread out over a period of time, you can get it down without exhausting yourself too much. It would still leave you tired, but not anymore than mundane physically demanding job that leaves you tired.