Hey!
If I understand your question and intentions correctly, you want to set your Launchkey as a control surface in the MIDI settings. Then, under MIDI ports, you probably have two Launchkey ins (one for the keyboard, and one for the control surface) and two Launchkey outs (ditto), and then a couple ins and outs for your MIDI interface (with one of those outs going to the Evolver).
Set the "Track" and "Remote" boxes to On for the control surface. Now you can assign clips to the pads, use the faders to control the mixer, etc.
Leave "Track", "Remote", and "Sync" Off for the other MIDI devices. This will allow you to use the keyboard to trigger Evolver notes.
Once those settings are made, close the Preferences panel and switch over to the Session view. Set the "MIDI From" dropdown on one of the tracks to the Launchkey's MIDI (the one that's not the control surface), and the "MIDI To" dropdown to the interface going to the Evolver. Make sure that the Launchkey's output channel matches the Evolver's input channel.
And that should do it!