Are chords basically DBX? If so, what are the most common motions for chords, is it usually forearm? And how does this change over chord size, e.g., 2 notes (not officially a chord), 3 notes, 4 notes, etc.?
I never thought much about this but it seems that USX/DSX are mainly for single notes?
(Sorry for such fundamental questions!)
Usx, upstrokes escape and down strokes are trapped.
Dsx, downstrokes escape and upstrokes are trapped.
Dbx, both up and downstrokes escape.
Do you mean arpeggiating chords? If we are talking 1nps then you’ll need a dbx motion.
If there are multiple notes per string, it could be a mixture of various motions, aka mixed escape.
Nope, chords! Funk, jazz, folk, etc.—not arpeggios.
I would be interested to know this! It seems like the typical approach for chord playing tends to be the flexed gypsy jazz form and an RDT motion which is mainly wrist with a little bit of forearm. I think you can see this demonstrated here nicely:
My guess would be that it’s a mixture of USX and trapped pickstrokes but I’m not sure 
Chords are rarely played fast enough for escape to matter, and being trapped doesn’t really cause as many problems because the whole point of playing chords is to hit multiple strings. In cases where it makes a difference, I expect it would depend on exactly what you’re playing.
Oh okay. Well I don’t think escape really applies to strumming in the same way it does for single note playing. I think what @Jacklr deacribes is likely the most common joint motions for strumming.
They are the fastest that I can think of in terms of notes per second.