Backfireing is or missing is an almost sure sign of being too lean with 4-strokes. If that happens at all, you are too lean, regardless of what else you think might be going on. Richen it up and see if that helps.

To get better pull, go to a larger diameter, lower pitch prop.

What prop range does the manual suggest? I think some of the props you listed are too small for that engine, but since I haven't run one of those myself, I'm not sure.

Also, MA props in that size aren't as efficient as some other brands, so you might try another prop.

I might try an APC 15x6 or 16x6 and see what happens. I'm running a 13x6 on an OS .61FS right now, which worked out to be the best prop I tried.

Also, I'm not sure what engine range is recommended for your plane, but at 10lbs, you may not get much vertical. And the Spacewalker isn't exactly designed for knife edge flight or verticals anyway. (not that it's not possible, but it's not designed for it. The Spacewalker is closer to a cub than to an Extra).