[Mike L]

The covering official will have to decide what action R21 is taking prior to the contact. From the post, there is no way for us to tell if R21 is also trying to catch the kick or is attempting to block or just how close he is to R20 ("vicinity" is pretty vague). All R's are protected if they are trying to catch the kick, but if he appears to be attempting to block a K player rather than field the ball he can be contacted. If there is some doubt about what he's stumbling around and doing, I would err on the side of attempting to catch the kick.
So, operating under the assumption R21 was a blocker and making no attempt on the ball, then you have to consider the block itself. If it's a hit with seperation created between K & R21 and R21 then stumbling into R20, you have R21 impeding R20 and R21 "blocking" R20 into the ball. Now, you have to decide did the ball go off R20 or R21. If it's R20, you are going to treat it like any normal muff of the kick. If it's R21, you can rule ignore the touch because he was blocked into it.
If it's a driving block where K and R21 remain together and both go into R20, I think you could easily rule KCI no mater which one actually contacted R20.
K has no rights to catch the ball before the touch because there are R players in the area. After the R touch, K can catch/recover and may not advance, but it could be first touching depending on the action above.
A Fair Catch signal would make me err a little more toward the KCI call, but otherwise everything else is pretty much the same.