Ahhh.... the joys of threading servo leads through a closed-in wing.
Used to drive me up the wall
Still does when you get one of those real tight stinkers...
I found that taping the servo lead plug onto the string with some thin electrical tape - making sure you form a bullet-head shape at the string end - then pulling the string from the other end works best. I end up jiggling the sting end while maintaining a bit of 'back pressure' on the servo lead helps to persuade the sucker through the wing ribs.
But go gently - whatever you do,
don't end up pulling the string off the servo plug once you've started threading the lead into the wing, or we'll hear the screaming down here in Oz

LoL
If you get a situaton like yours where the plug is a really tight fit through an opening, you can always try to remove the plug itself to leave just the bare wires, thus reducing the size of the blob you're trying to thread through the wing. If the plug is the 'moulded on' type, you can't do this, but a lot of servo plugs have those tiny gold tabs you can carefully pry and release the wire + connector spade from the plug itself.
If worse comes to worst, I've cut the darn plug off... fed the wire through...then re-soldered the plug on.
In some real b@stard installations, I've found that taping the string/plug to piece of control-rod allowed me to pull with the string from one end while pushing with the control rod from the other end helped.
Those things can be soooo frustrating to get though a wing sometimes - and all it would take to make it a doddle would be for the guys making the wings to make the openings in the ribs a whole mm bigger. But sometimes they don't
Re your motor screws, not sure of the motor you're running but 'M3' (3mm metric) screws are fairly common-place for Chinese and Euro motors. American stuff sometimes uses imperial (like a whole set of barrel-connectors did on one plane I have... bugga).
Hope this helps...
BJ