mimic's not only met wolverine, his out-of-control power turned him into an exact physical replica of logan for a time. the villain from deadly genesis isn't mimic, unless continuity's been thrown in the trash.
also, the third summers brother is not gambit. the "sinister clone with summers blood" idea is specific to 'the end' alone, and has absolutely no reflection on canon as we know it. adam x was intended to be the third summers brother, but the idea was dropped around the time x-treme x-men started, as having a character whose name was the same as another book would've been too "confusing."
the villain may or may not be the third summers brother. i believe it was only said that the issue of TSB would be cleared up in deadly genesis, not that the villain was, for certain, the third brother.
also, some background history, TSB is an idea/term made up by the fans. the only canon information at all relative to the idea is when mister sinister said he'd been watching cyclops' "brothers" for some time. the plural was made note of in the comic, so it wasn't a continuity error. for all we know, there could be seventeen brothers, though the TSB idea seems to have made its way from fandom into the mainstream comicverse.