Just a couple of comments. 0.6 wire is fine. CO2 is fine too, different to weld with, not as easy, will probably need higher voltage than using a mix gas. I used 0.6 and CO2 for many years on bodywork. Welds will be taller and need a bit more grinding but that's OK.
Welding in different positions as you describe makes a big difference, it sounds to me like you need quite a bit more wire speed.
Try on some scrap, in the same position and while Welding with one hand turn the wire dial with the other, just to experience what happens.
One more thing, your multimeter doesn't mean much when checking earth path. You don't just need continuity, you need to be able to pass a lot of current.
If possible clean some of the same panel and clamp to that, when that's difficult i often weld on a tab to clamp to and take it off later.
Welding in different positions as you describe makes a big difference, it sounds to me like you need quite a bit more wire speed.
Try on some scrap, in the same position and while Welding with one hand turn the wire dial with the other, just to experience what happens.
One more thing, your multimeter doesn't mean much when checking earth path. You don't just need continuity, you need to be able to pass a lot of current.
If possible clean some of the same panel and clamp to that, when that's difficult i often weld on a tab to clamp to and take it off later.
