The instruction pictures show soldering irons, a roll of servo wire, and some scissors on page 2. Look at the top of that page. What they suggest is actually the best way to deal with getting the servo wires through those wings. If you cut the existing servo lead and solder an adequately long section of wire to the servo, thread that through the wing, and then solder the original end you cut that has the connector, that works well.

If you have a crimper and replacement connectors, that makes an even cleaner job of it. You'll discover that it's worthwhile to have just enough connector/wire hanging out of the wings. Where they can get into the fuselage has restricted room in that area. Wire splices cause a bit of problem when stuffing the wire in where the two elevon servos are and where the back of the LiPo might need to be for balance. The Avia is made to purpose and some things take a backseat.

As for the elevons. No, Y connecting the two servos won't work.

BTW, about the elevon pushrods..........