FCPX: Problems using Twixtor

I am editing a project with mixed 2K and 4K clips. Everything is transcoded into Proxy Media. This project will have a lot of slow motion sequences. Normally I was using 2K clips with Twixtor (no Proxy) and did not have any problems at all. I have the following issues:

- If I apply a 50% slow motion on a 10sec clip, the resulting clip should be 20sec long. But it stays 10sec long. Means I loose part of the clip. Means, it does not give me the right duration.
- If I trimm the twixtored clip after it has been rendered (let's say I shorten it), I have to re-render it. The orange bar appears again. This happens with any modification I do to the clip.
- The rendering process takes more time with proxy than without proxy.

Is there anything I can do? Cannot find anything helpful out there.
Thanks a lot for helping.
Lucian