A few thoughts.
First, you'll likely want a different arrangement for the monitor speakers and headphones than simply splitting them from the line out. When recording you'll want to hear computer playback mixed with the new material in your headphones...but only feed the new material into the PC.
Second, unless you can a very professional (read: expensive) mixer that can act as a control surface via MIDI control, you won't want that link.
Third, you don't show a separate audio interface to handle the line in from the mixer--but if you're assuming on using a basic inbuilt sound card, don't. You'll need better for recording use. On the other hand, if you're talking about a specialist PCI interface in the PC, then you should be okay.
Fourth, you'll want line out from the PC to spare inputs on the mixer, again for monitoring.
Fifth, all the line in/line out stuff to and from the mixer might be replaceable via USB or Firewire depending on your mixer choice.
As you may be realising, if you go with this generic setup, a lot is going to be determined by the capabilities of your mixer.
Hope this helps.