Newbie needs some very basic help

NEW to Rhino and looking for three very basic “how to’s”:
1.) how do I draw (or edit to achieve) a circle with a radius of 1.25679"? drawing a circle from a grid point with a radius defined by another grid point isn’t the problem…
2.) how do I define/establish a non-standard working plane?
3.) does Rhino have a “marker” for non-grid points of interest?

In general, I’d recommend you go through the tutorials in the User’s Guide, found on the Learn page.
All of these are covered there.

For the circle, start the command, pick a center point, type the radius, and tap Enter.

For the custom Construction Plane, start the CPlane command and tap the F1 key to open the Helpf file article for it.

For the last one, I think you’re looking for the Point command and will probably need to use an Object snap, or the SmartTrack tool