Hi Adam, don't feel bad , you haven't messed this up rather spectacularly , I have messed it better sometimes . about your thighs, If i can make some suggestions:
1- for alignment of the thigh you must start from bottom to top and them trim the excess on top.
2- It is always recommendable to make the first fit using painters tape or some tape before any cut.
You can gain some more room by removing all the return edge of the thighs, taking care of no cut the bottom ridge
What i certainly would do, Is first, try removing the return edge, make a fit test and look if the shims are necessary. You can also make an internal cover strip wider and leave more space between both pieces and then hide them with the external cover strip.
Hope I have been clear, it's difficult because of the language.
Try to post photos.
Cheers