Yea, there is a weakness, it is hard to use swift through mid-air as its full strenght is achieve while you run on the ground thus if you are facing a swift user, you can find a way to alter the ground that they would be running on. Example is the swamp of the underworld, they can't run on this, spikey terrain and many more.
Nah, i don't think you can infuse weapon with swift, not just yet, maybe an old cj may, dunno who has it.
Good!
Any questions?