I need some new bike shoes PDQ as my 5.10’s have died in their sub-yearly cycle.
However.
I’m currently a flat pedal rider thinking very strongly of going to clipped in. Therefore I could do with a shoe that will work with flat pedals for a week or two before using SPD’s. Don’t have the pedals yet and want to tackle Innerleithen “secret” trails using a platform i’m happy with.
Can I use the AM45’s in this manner?

I wouldn’t. Different sole to the AM41 and I think you’ll get much less grip. You’d be better off buying a cheap pair of skate shoes and some proper spd shoes.