Looks like the 2x12 goes right up to the plate omitting the need for cripples which would have been fussy with the amount of space left over if you used a 2x8 header. Also that extra 2x king stud could be a used as nailer for the fin on the New window.
Metal studs are fine just a lot of screwing. I mentioned wood because it’s easy to frame. Frame it flush with the existing steel structure, on that corner just run your sheetrock/wood/other over the Metal structure. You can anchor Your new wall material into the metal studs On this side of the...
Do 2x4/6 Wood infill framing flush with the existing metal framing. Run a 2x sill and top plate for the studs on top/below the existing metal framing. You would need to bolt/screw those to the existing metal frame. Now you’ve got a lot of flexibility to finish the walls with Sheetrock, plywood...
Mercury power tune most boat shops have it. It’s specifically designed to break up carbon. You need something to break the carbon up I don’t think injector cleaner would work. Google stuck glow plug you’ll get a ton of hits.