Mach3 posts will usually work.
Yes, mouse click or enter for MDI. I use Enter, as you need to hit Enter after typing anyway. How else would you access the MDI?
No, G53 is not Modal. It must be on each line.
No, one feedrate only. Not sure what purpose multiple feedrates would have, as the slowest would have to always take precedence, no? That's kinda how it works, as if your feedrate was faster than any given axis' velocity value, that axis velocity would limit the feedrate.
You can only use pots on the UC300 controllers, which have analog inputs. You can however, use an encoder, but you'd need to write a macroloop to do the work. It's not a built in function.
You can assign the spacebar to pause on the Hotkeys page of the screen.
The spindle speed starts at the minimum. Not sure there's an easy way to change that.
In my custom screen, the jog speed and increments are remembered from the previous session. I don't think you can do that without making some screen changes.
Get out your physics book. Wouldn't inches/sec² be 1/60 of inches/min/sec?
1G = 9.8m/sec², which is about 386in/sec².
According to the calculator I looked at, it takes .1sec to go from 0 to 400ipm if the accel is 800inches/sec²