From looking over all the posts, I suspect the glow plug "that the previous owner had in some stuff" is possibly a marginal plug at best. I would purchase a new OS A3 plug and put that in the engine.

Second, you never mention which prop you have on the engine. It could be either under or over propped, which will also have a dramatic effect on the ability to get the engine properly tuned. Too large a prop will tend to make you want to run the engine too lean, leading to overheating, and exactly the problem you described with short runs at WOT. Further, the lean runs will greatly shorten the life of the glow plug. Too small a prop and you'll peak the rpm too high, again possibly leading to overheating and shortening of the plug life. For the OS-46 AX, try an 11X5 or 11X6 APC prop on the Nexstar.

As for the engine not being mounted straight, this is normal. The Nexstar has a fair amount of right thrust built into the firewall.

There have been many posts on engine tuning. Getting the low end properly set is an art that may require a bit of trial and error. Make small adjustments, and reset the high speed needle after every low end adjustment. Then use the throttle response to tell you if its correct. Its often very difficult to tell the difference between a rich or lean low end setting. Slightly rich hesitates and then advances to WOT. Too rich and the engine simply sputters and dies. Too lean, and the engine will sputter and die when the throttle is quickly advanced. Use both the throttle reponse and the pinch test to help you figure it out.

Good Luck!

Brad