I have built 2 of these planes and love them both..first off this plane is for 3D..DO NOT get it too fast it will flutter the ailerons right off the plane...don't ask me how I know.
First question...the 4 plastic bolts are enclosed to mount the wing of course..you only need 2..the other 2 are spares if one should break..do not tighten these down to much just "snug" them up,under that thin balsa on the wing mount is a piece of hardwood triangle stock that will not break when screwing in the bolts but I just put a few drops of CA on the balsa to harden it,2nd question..I have NO idea what the square pieces of balsa wood they included where for..mine are still in the box,3rd..if the control horn is mounted per instruction the horn will come nowhere near the fuse when deflected..look at mine,one thing though,I can only get about 42mm deflection out of my servo..they call for 50mm each way..I have no problem with the 42mm.
I sure hope this helps!