Dianna,
A couple of things to bear in mind.
Place of birth may not be place of baptism.
At Mill of Garvock suggest he was working there, they could have moved, you have to be a bit flexible some times.
I use Ancestry quite a lot to do suggestions and then fact check, you could use any of the sites in a similar way. I have found quite a lot of people in places I would not expect, Minister from near Carlisle, England getting married in Dundee, Scotland. But it was definitely him, was recorded as such.
Have you tried building a tree back from them? There seem to be a few Charles families in Garvock, you might find Alexander that way.
Also John’s father should be an Alexander, and there is an Alexander baptized in Garvock at about the right time?
Or John to Alexander in Insch.
It can bc a case of building a lot of mini trees!