What people keep forgetting is that a) nukes aren’t that dangerous, not at the sorts of distances and scales we’re talking about and b) the Orion drive’s pulse units are very small and low-yield, 1-5 kilotons at most. The EMP radius at blastoff is on the order of a few hundred kilometres, and as long as you’re careful with your choice of launch site (high latitudes, so the small amounts of fallout it does generate don’t get drawn up into Terra’s magnetic field and generate an artificial radiation belt) and have a slab of graphite underneath you when you’re lifting off from the surface (to minimise the ground fallout), the environmental impact is quite manageable. And besides, it’s not like anyone would miss southern Australia or Greenland all that much.
Oh, and just keep dreaming about using FTL for cross-system travel. All of the theoretical systems that might work without snapping the rest of the laws of physics in half are the sorts of things I absolutely wouldn’t allow within half a lightyear of anything inhabited, because half a lightyear would pretty much be the minimum safe distance if they decided to go boom. Even the relatively thrifty warp drive concept Harold White et al. have come up with (“only” 700-odd kilograms of mass-energy, so enough to sterilise a continent) still fires a biosphere melting blast of gamma rays whenever you turn it off, so it’s hardly safe. Unless someone much more advanced than us gives us a safe FTL drive (which would presumably be a mass of safety systems with the actual drive sort of hanging off of one end), I’d imagine the minimum clearance would be enforced by the biggest guns at humanity’s disposal.
As for Tabby’s star, the current preferred hypothesis seems to be that it’s in the process of eating one or more planets and that the dimming is caused by the debris field produced by planets getting violently ripped apart by tidal forces. Which is really, really cool, and a lot more likely than the Dyson sphere hypothesis. Really, Dyson spheres are very, very expensive things (even the statite swarm model, aka the only one a sane engineer would build, along with the orbiter swarm model), the sort of thing you don’t build unless your other energy sources can’t keep up with the demand. If it was a DS, we would have seen plenty of other energy emissions, high energy stuff like antimatter (antimatter drives on starships in particular would have some very distinctive visual signatures), big fusion drives, flashes from launch lasers firing relativistic ships out into interstellar space and other signs of an active high-energy civilisation. Even the least civilised options, like an N-D beam station (aka a Death Star) being built by Von Neumann probe berserkers or something along those lines, would still be showing up as a lot more weirdness in the star’s emissions than just some odd dimming, I’d suspect.