Hi plish, also it looks like another way you can do it with your previous "twist first" method is to delete the top and bottom faces after you've done the twist and then select the now open-ended twisted box and use Construct > Planar to close off the ends with brand new planar end caps.

Probably after that the Inset will work - basically what happens is that after you do a control point manipulation on an object all surfaces that make up the object are converted from any "analytic" surfaces (specialized surface classes for specific shapes like spheres, cylinders, and planes), and instead become just a generic NURBS surface. But some processes like offsets work better when they can use analytic surfaces when possible. Deleting and putting new end caps on will make the ends to be analytic planes and that will help, and also saving the file and reloading it will cause them to become analytic surfaces as well.

I can probably fix this up so that after you do some control point manipulation any things that are planar after the point squishing can get set up with analytic surfaces again instead of those being lost as part of the control point editing process.

- Michael