Unfortunately, SE/Yoshida disagrees that THF would be redundant from a design perspective. Not only does the new storyline hint at it coming in, Yoshida even said that THF was one of the first new jobs he wanted to bring in, along with things jobs like Red Mage, Blue Mage and Samurai.
RE: Archer -> Musketeer; it's unlikely that will be case, even if you don't think so and are basing this on your own thoughts of what should be (all subjective) and not what is likely/speculative (a mix of subjective and objective). Archer will likely go into Ranger, which would be a pure DPS branch from Archer, and Bard will continue to become more and more support/dps hybrid as the level cap increases. Now, I'd be quite happy seeing Ranger be a pet type class, and that would work quite well.
That, or further the field/trap based abilities like Flaming Arrow.
Musketeer, however, is a class, and can be seen as such because it has it's own guild in the Coral Tower (which is now also home to the Marauders).
How weapons will come into things, I have no idea. But I'd expect a gunblade type class/job to come out of Garlemald, as part of an expansion that goes into the heart of the Empire. The technology of a gun/weapon hybrid seems very much theirs.