26 down is definitely "moho" to fit in with 25 across and 29 across ("Omega" and "Shrimp").
Moho is short for "Mohorovicic discontinuity" according to my trusted Collins' English Dictionary.
Keyboardix is "pustule sur" with moho, (pardon my Franglais). Be careful about the spelling of 4 down, however. It can be CAY or KAY. Do a Google and you'll see what I mean. 50/50 chance.