Michael, I had the same experience as you regarding the upper wing of the dreidecker. Hot water and bending apparently work, painted the wing only to find some days later that the wing showed the exact bend... still have the kit shelved waiting for energy to solve that. It’s a shame that an otherwise beautiful kit has such basic issue

G'day George (@gblair),
Unfortunately these wings are single pieces, so your excellent first solution doesn't apply.
The single piece gives a great trailing edge, but I suspect the upper wing geometry - convex chordwise and concave sections spanwise - creates tension in the plastic that causes the problem.
I'll try the hairdryer again.
Failing that, I'm tempted to try your second idea. ;~)

So, a little more progress.
First the wings.
I tried again to straighten them by heating them with a hairdryer.
Again, they straightened only to revert to a slight bend.
The first two photos below show the top wing - original and final state.
They aren't straight but I dare not try again because I think I caused a bit of damage as seen in the last photos.

4 attached images. Click to enlarge.

Now the fuselage:

Fit was fine on the upper surface except for a bit of miss alignment a the rear, a slight gap near the cockpit and a sink mark on the rear starboard surface.

Fit on the under surface was problematic.

The halves have a slot for the centreline stitching piece moulded on each edge (half each).

Unfortunately these were of different depths so that they didn't meet neatly. Plus, I think that the slot is not quite wide enough.

I tried liquid glue, but the parts didn't form a strong enough bond and internal pressure forced the sides apart.

So, I used super glue and added the centreline piece.

You can see some fine gaps that I will try and fill, otherwise it is not too bad.

2 attached images. Click to enlarge.

Not progress but...
I saw this on ebay and decided he will look good standing next to his aircraft - Lothar von Richthofen.
I was a little alarmed that he has no hands until I realised that he has them in his coat pockets.
I only hope I can do him justice.

2 attached images. Click to enlarge.

Wings look much better, Michael.
And I agree with Spiros, @fiveten, that Lothar will be a great addition to your Dr.1

This is how the turned out after the masking was removed.

2 attached images. Click to enlarge.

I may do a bit more blending, but first I have to mask and paint the wings.
I only have a couple of weeks to get this bird finished (8th of August).