When I go to pull servo wires through wings, I usually tape the string to the wire.

If the string comes out with out the wire, no need to panic. Get a longer string, tie a small nut to it, hold the wing vertical and you should be able to let gravity pull it through. When pulling the wire through, it often helps to hold the wing vertical, wing tip up to let the weight of the connector help things. I've also used a very long servo extention (36") and fed that through the wing from the root outwards. Then connect to the servo lead and pull it back through. Disconnect the extension when done. The extension holds well, and is less likely to get snagged than a knot or even tape.

When you're setting up to epoxy anything in place, you should always fully assemble every part "dry" and make sure everythings fits right. Then mix up some epoxy, the slower setting the better. For wing centers, I usualy use 30min, if not longer (depends on what I have in the shop). The longer setting time gives you time to carefully put everything together.

As John said, if the epoxy is wet, a big of alcohol or paint thinner can get it off easily. If it's dry, you're pretty stuck. A big blob will often fall off if you heat the covering with a heat gun and kinda flick at it, but a film or "finger print" of glue is often there forever.

In general, slow down and take your time. ARF assembly can go really fast once you are familiar with how these things work, but people who are not familiar with how these things are done usually take many times longer. That's ok.