No matter what you try to modify the bolt to turn it out--i.e. welding, slotting, two nuts etc.--you will have to first address the problem of seizure/binding or you will just be swimming uphill. I've had OK luck with lot of heat and patience, but let's face it, this isn't an easy bolt to get to. You won't be swinging a mini sledge against a pair of huge vise grips here. Drilling, filling and tapping doesn't sound that efficient to me either, honestly. I'd be tempted to stop now and take swingarm to a good machine shop. Probably lot cheaper than a new/used swingarm.
If your town is too small to have a machine shop, go to a bigger town!! Or you can always look online for a used swingarm and set this one aside for repair later.