Hm, isn't it kindof interesting that to verb the noun, the "bu" is changed to a "ru"? I mean, why not use ケバぶ as a verb directly.

That's an interesting point! It's evidence that the rule is "create a two-mora abbreviated morpheme of the loanword, and add -ru" (which my chain of derivations elides -- I will fix that) tather than a more formless "do what it takes to make sure it ends in -u, using -ru as a default ending if necessary."

True, ケバばない does sounds wrong, somehow, doesn't it? I guess the low-hanging hypothesis would be that it's a loan-word handling rule akin to the [Chinese] + する of old, but I wonder... it seems to be used for verbs from onomatopoeia as well: パクる、チクる、ビビる等 Maybe it's the default verbolator when you can't/don't want to go the aux. する route? Or alternatively, for words that allow hypocoristic shortening (unlike Sino-Japanese)?

Default verbolator makes sense to me- actually there is a line of argument that /r/ is a "default consonant" in Japanese (I know LaBrune published something about this recently), which would also fit.

Hm... /r/ can be analyzed as a "default consonant" in 一段 verbs, but these verbs are all 五段. It definitely feels like a morphemic -r- to me here. There seems to be a similar phenomenon going on with ラー as in マヨラー (slang for somebody who likes mayonnaise), which is obviously influenced by English -er, but where this -r- also jumps in.

Aren't all those /raa/ words basically analogies from /Amuraa/, though? That gives you an etymological source for /r/ in that case (Amuro as in Namie).