Locke is a wackadoodle of that there is little doubt...Charlie said "he will save us all". Remember early on Locke appeared to meet up with the monster, live to come back the next show and say he had "seen the heart of the island"...
As for the Shannon and Boone thing...she referred to Boone's mother(the one who inherited all the money which Boone is spending)as "your mother" so hopefully things are not the way they seem...not that it would be a surprise either way on this show!
