I generally don't go rewiring rental houses to provide 240v in what everyone else will assume is a standard outlet - that's not a habit I've gotten into, and not one I intend to get into. In this particular case, I'm fine with relocating the triac and welding slower.
If I understand your modification properly, you're taking a standard two wire (hot, neutral) plus ground setup into a room, replacing the neutral with an out of phase hot (for a 240v difference between them), modifying the other outlets to use the ground as their new neutral, and then tying the ground to that neutral. You've now made the existing ground the neutral, and there is no proper ground anymore.
I don't argue that it works, in terms of providing proper voltages at the outlets. I understand that it works.
I'm arguing that it's not a good idea (actually, I think it's a truly terrible idea), partly because you have a 240v outlet that a 110v device can be plugged into (not everything with a plug is an auto ranging switching power supply), and partly because you're consistently dumping line voltage into a ground wire instead of a protected neutral wire, and this can lead to significant potentials on exposed metal if everything isn't perfect.
"Doubling the load capacity of already installed wiring" is not a good idea either. It's spec'd out to a certain gauge and design for valid reasons, and your modification tosses those all out the window.