Hi Steve, well here is an idea for a much different approach.

I took your curves and moved them to center around the origin, it is kind of a good idea to use the origin snap to make sure things mirror and align exacty.

Then I took the curves for the top curvy piece, and drew some little extensions on them, so they kind of extended further. I also used Transform / Scale / Scale1d to kind of stretch things out slightly. I did this to get this kind of setup:



So the idea here is to create a much more regularly shaped outline for creating the top surface. It can be difficult to construct a smooth surface directly off of curves where one curve is kind of bending and contorting quite a lot more than the other - the surface construction commands will tend to create bunched-up type surfaces in that kind of situation.

By building a kind of extended curve frame you can make the curves a lot more similar in shape, this will tend to make smoother more controlled surfaces.

So after extending those curves, I used Construct / Network to build a surface there:



Then I took the bottom outline which is all one closed loop, and extruded it upwards:



Now select that big block piece, and use Construct / Boolean / Difference and pick the curvy network surface as the cutting object. This will slice the block into 2 pieces, discard the top piece and you're left with your mouse shape:



This one is not quite perfect, it still needs some tuning in the network surface to smooth out some of the curves so the network is not quite so wiggly. Less wiggles will make for a cleaner edge curve where the 2 pieces intersect.

So you can see the general idea here is to not try and follow quite so exactly along every single digitized contour of the final 3D mouse. Sometimes it is much more controllable to extend your curve framework so that it can be more simple and regularly shaped, and then use booleans or trimming to cut things back to a more wavy edge.

With this method, you kind of give up the exact control of the edge where the 2 pieces meet, instead of drawing that directly it is the result of the intersection between surfaces. But because you deal with a more regular network your surface quality will be greatly increased...

Model is attached here as mouse2.zip

- Michael