If the right tailor tapers the Coast through the inseam it should not mess up the fading. A skilled tailor would match up the fades, and assuming you aren't taking it in more than .5" it should be fine. Probably would not recommend hemming them, though.
As for stretching, they will stretch a bit. Not as much as the raw, because they are pre-washed, but the denim used for Coast in its raw state is actually the same denim as Cobalt.
@JE + co...what about the fit on the graphite vs the obsidian? I've found the graphite to be more snug in the thigh. Just curious how they run by comparison.