I didn't have a chance yesterday to get back to this thread, but you're on the right track, for sure. (John - That's the picture of the console I was thinking of.)
There are numerous ways you could make a sliding shelf for the mixer. The cheap way is to just build the recessed area for the keyboard with a lip that is flush with the bottom side of the desktop. Then you would just set the board on the lip and put the mixer on top. This would allow the board and mixer to be slid back and forth above the keyboard as desired.
Another option would be to recess some drawer slides that the shelf would attach to and provide a nice, low friction option.
When John talks about getting centered he's referring to being centered on the wall where you are mixing.
As far as making your desk solid goes, try to make the lower sections nice and square, use glue and screws to attach the pieces, and use a piece of plywood or MDF on the back. The back piece will make a flimsy feeling frame into a rock solid cabinet. Then you just attach the top. On the back side, also make sure if you use plywood or MDF to cut some large holes to feed in cabling for your rack gear and provide airflow for cooling.
It seems like a lot of work when you plan it out, but once you finish it, the time spent will be well worth it. You have a great start to what will become the heart of your studio, so have fun with it.
Darryl.....