Any question answered...

I though the IS bit meaning International Standard means they are all the same, most brakes don't bolt directly onto the IS mount, although some do, I would have thought the size of rotor in these cases is directed by the whims of the caliper designers. When you get down to 140 I'm not so sure as it's teeny tiny. The main reason I'd have thought the PX cheapo ones won't work is due to the dip in them not being deep enough to fit that specific caliper in.

Post mount calipers don't care about rotor size.

IS mounts are standard, you are right - 51mm between the mounting holes. I thought distance from axle could vary, internet isn't sure.

PX Post to IS calipers won't work, because he needs IS to Post.

Adapters are caliper design specific, you are right.

Shame nobody makes prefab Post mount drop outs for artisan frame builders but hey, that's life.