And I need to install/change the housings for the two derailleurs.

How to do you stick them through the frame and catch them through the opening in the front?

Edit: Mission accomplished with the help of a piece of string to keep the existing housings (rear brake + light) out of the way as much as possible, and a light + tweezer to reach and pull the new housing out. 👍It can then be used to run the second housing (front or rear derailleur) through.

Edit: Because the bend is very tight to route the front derailleur housing before it enters the clamp from below, you'll need to add a flexible noodle (for lack of a better term):