What is the recommended CG location? Where have you had it when it was flown?

What engine is recommended and what are you using? what prop?

I wouldn't worry about the leading edge shape. One of our regular fliers at the club field builds his own planes. On his last one, he didn't shape the LE pieces. They came as 1/2" square sticks in the kit and that's how they were when he covered them. It's a Butterfly. They have a very large glider like, polyhedral wing with a LOT of LE. It flies way better than anyone would imagine.

I'm not sure what your question is about the covering on the top ends. You've got a couple of pictures that seem to show some wrinkling in the covering in the last open bay. That can be removed with heat. It shouldn't be significant from what I can tell from your photos.

The design of the airplane uses a low aspect ratio wing. Their characteristic is that they are pitch sensitive in that when pitched a certain amount they create appreciably more drag than would be induced by a wing of higher aspect ratio of similar area. What that means to you and that Slow Poke, is that it will probably appear to be elevator sensitive. Much pitch change and the sucker is going to slow down.

A couple of us experimented with wings of about that aspect ratio back in the 60-70 in control line combat. The lower aspect ratio wings were lousy in fast combat because their speed was killed every time we tried to turn them. They felt and looked like they were stalling (they weren't) and the engines had a real problem getting them back to speed. The perception of what they were doing in flight could have been the same perception you're getting.

The parallel between those combat wings and your Slow Poke could be that the Slow Poke actually has marginal power for the design. The design is going to give huge drag at times. The engine isn't going to be a powerhouse. The power loading is going to be on the low side for the drag the airplane can throw at it. 15s are not big prop turners. They turn little props fast. That's a recipe that works well for little fast airplanes.

I'd lean toward trying to put as much fan in front of that plane as possible. Larger diameter prop with flatter pitch would be the idea. To swing that, you might look around for a bigger engine. Or simply try the largest diameter, flatter pitch props suggested for 15s.

Next thing to do, and it could be the most important, would be to limit your elevator throw. That wing is going to give you most of it's best performance in a fairly narrow range of Angle Of Attacks. You're not going to need an elevator with more throw than what's needed to get that range of AOA. It's possible that the available elevator movement is getting that little prop on a little engine on the front of a very pitch sensitive wing is the cause of your problems.