These are all great points that are new to me. Wish I knew this when we started the wing because the leading edge was slightly bowed.
Don't worry about it, you are doing fine. At Tim notes, you can always twist it while heating it once it is covered to remove any warps. This style of construction makes it nearly impossible to keep it straight. Since the covering it nearly the only thing holding it, even if it was built perfectly straight with no built-in stress, even a slight difference in the tension while covering it will cause it to warp then. And it is not going to be particularly stable from day-to-day, either, with whatever warp it wants to have tending to come back even after you twist it to correct it. It's not really avoidable with this style of construction, tweaking it periodically with a heat gun it just part of it.
If you have solid parts like spars, and they are not straight, there are multiple ways to go about fixing them before (and after) assembly. For instance you could make up your 1/2" square LE stock from two lengths of 1/2" triangle stock, put them together and push them up against a ruler while gluing them. Or, take the original piece, split it from corner to corner, block it straight, and glue it back. It is very hard to find a single piece that is straight in all directions, building them up allows you to block them straight. The trailing edge stock on my current airplane is built up of *5* layers, and while none of the individual bits of it were remotely straight (like the .020 balsa down the middle, the resulting glued-up piece was as straight at the 48" straightedge I used to jig it up.
Ted Fancher and I assembled an ARF Strega one time. One of the problems was that the fully-finished wind had a warp, TE up at the tips. This wing was built a different way than yours, and would not easily "tweak" with a heat gun - way too stiff. Solution - take a long thin carving knife, slit the entire trailing edge horizonally from about half-span, all the ribs about 6" deep. and around the wingtip. That allowed the top and bottom half of the TE able to slide sideways, tweak it, Ted held it straight while I put on the thin Hot Stuff and pushed it together. in the new position. 2 minutes, straight (or straight enough) wing.
Brett