A few years ago I had to drill out a frozen Allen screw that was keeping the mast from turning. It took days of patience, PB Blaster, an etching tool’s vibrations, and drilling care to get the rusted-in screw out.
I’m not sure from your photos if it’s the same Velvacs since I can’t see such screw covers, but they otherwise look the same as mine, or very similar. If they’re the same, did you remove the plugs and try backing the Allen’s away from the shaft? The original plugs have slots but care must be taken with a screwdriver. I ended up at Ace Hardware getting pop-in (not screw-in) hole plugs as replacements, after repairing the frozen Allen situation. The replacement plugs aren’t rustproof over time, but don’t cost much, so I got backups.
Joel