I guess I should have known it was coming. Just because I have been successful with my last few attempts at pant drafting it doesn't mean that those methods will work on a completely different design in a completely different fabric. The front looks okay (apart from the wrinkles, but that just the fabric). The back however, does not.
I'm very glad I did a muslin.
Making a pant muslin out of a light weight cotton/lycra shirting is however a bad idea. There is no shape or hold in this fabric whatsoever and it wrinkles terrible.
Even though the fabric is partly to blame, there are a lot of other things wrong in the back. I don't know where to start.
It looks like my design and construction ideas will work. I also like the shaping of the pants and they fit perfectly around the waist and knees.
The crotch/butt/back thigh area, it's horrible, horrible! I hope that someone can give me suggestions on how to improve the fit. One obvious mistake was that I used a crotch draft for a regular straight leg pants, that clearly doesn't work on these fitted stretch trousers. The crotch is not extended enough for instance. I should have followed Mrs Stylebook's crotch draft instead, which is also similar to the fitted jeans pattern that I have. Also my poor choice of muslin fabric probably makes it look even worse. I'll make the adjusted muslin in a more stable fabric.
The side looks a bit better, but still suffers from the same problems as the back, i.e. loads of twisted excess fabric.
The button placket solution will most likely work, there is no gaping involved, which is probably due to the interfacing and the close placement of the buttons.
It looks like I won't be able to topstitch the placket since it distorts the lycra fabric. I hate sewing lycra wovens! I'll try machine blind stitching instead and hope that will work. It doesn't look like there is any topstitching on the original pants either, so I assume that it's done in a similar way.
I'm a very stubborn person. I will pull this off.