Is there a "speed" (e.g. 1.0 arcsec per year) that defines "high"
proper motion (as opposed to low or medium)?

Limiting to magnitude 18 or brighter (i.e. within reach of most
amateur CCD gear) have all the high proper motion stars been
catalogued, or do any remain to be discovered or at least have their
motion more accurately measured?

If I find a star in one of my images that's moved a lot from its
position on an earlier DSS plate, then how do I find its "known"
proper motion to compare against my results?

A good example is the mag 14.5 star currently at 19 53 24.51, 11 36
07.4 (near NGC6837 in Aquila), as that's what prompted me to ask these
questions.

Is there any scientific value in pursuing and recording these beasts?
Can the amateur contribute as in variable star observing?

Thanks in advance for pointers and help…

Cheers
Beats