@aceflor: Are you sure that's actually the proper name fontconfig sees CharisSIL as? (i.e., what name does it show up as in the KUAL Font Override menu?). Space and casing has to be exact.
That's the only thing I can think of that would make it pull the default sans font instead...

. IIRC, the handling of Palatino is pretty straightforward, so that should definitely work.
And yeah, I really, really, really don't recommend touching either sans, Helvetica or Futura

. Although it's possibly becoming less of an issue on 5.7.x (for Futura & Helvetica), since they've switched to a different sans font in the UI and neither Futura nor Helvetica should be used anywhere in the UI any more.
But I still have stuff to check on FW 5.7.x, so, not ready yet

.
EDIT: The latest snapshots should handle FW 5.7.x better, and as mentioned in the previous paragraph, the sans override is now safe (on FW >= 5.7.x, that is).