1. As I recall, you actually could do that on the Inter-Tel 4000 cordless phone. Product discontinued long ago, but you can still find on secondary market. remember that this phone uses the old Inter-Tel wiring (orange/white pair) instead of 8528 wiring (blue/white pair). Cute trick: because of the wiring you can plug both a new phone and 4000 into same jack using a simple y-splitter.
2. Not really, but maybe use silent monitor and barge instead? Or Agent Help, whichever fits best.
3. You can program whatever digits you need under station speed dial buttons, no spaces needed. But you're limited, 16 characters I think, so keep password short. So this button for mbox 123, pass456, would be typically 2500*123456#.