Being lazy. I'd look long and hard at that bolt and my best, cleanest edged and sharpest toothed vise grips. If I went for it, it's a one shot deal. Side of grips against the dropout, a hard, bolt mashing squeeze, then turn. Might start by center-punching the bolt. After the visegrips, that might be difficult but pre-punched, you might be able to start a drill for an easy-out if the vise grips fail.

Edit: I see you've tried visegrips but not as aggressively as my approach. I'd squeeze until the teeth are beyond the threads and into the bolt shank. Ie, nothing subtle! And this calls for good vice grips. If yours aren't , don't.