Anyone know if you can disconnect the cable from the decompression valve (engine end) without the cable coming off the lever inside the clutch cover on the kickstart end? 
if so, how?
I want to do a compression test

if I recall, the cable attachment end is screwed in to the block aligned vertically w/the lever hook (page 77 service manual), thus will ~likely not come off. I think you are definitely ok to do a compression test, would not ride it this way.

Or maybe pull slack out at head end of cable and put a piece of (my favorite) blue painter's tape to hold it extended. First crank, will pull min slack for lever pull distance. Then at least that may minimize the amount of cable ready to go to war w/plastic tach gear.