A variation on the above is to slip in a piece of thin sheet styrene, edgewise.
put masking tape on either side, then carefully trim and sand it down.
I've never had any success filling gaps with CA, but thats me.
I also can't stretch sprue worth a dang either, so that leaves me with few options.
Any time I do joint fill, I mask pretty close to the gap. Otherwise it just creates more work clean up.
My go-to putty is Bondo Glazing and Spot Putty. It's one-part, comes in a toothpaste tube and is easy to work with. I also like the Tamiya light gray stuff.
You can try gluing the top half of the wing to the fuselage first, then add the bottom half.
Bill