you can definatley do that. the opus can turn on the laptop, but then the opus needs to know that the laptop is actually on, or it'll repeatedly try to start it, causing it to shut down soon after starting. the startup shutdown controller portion of the opus needs to know when the pc is on & when it's not for it to function correctly. an atx style board does this through the atx connector, you can do this with a 5v relay tapped into one of your usb leads, then use this to trigger the opus to stay on.
the 19v stand alone ps can be triggered from a relay off the opus 12v supply rail but to actually power it from the rail is unecissary & a waste of good clean regulated 12v, the 19v ps doesn't need a regulated input as it does it's own regulation, so straight battery power is good enough for it. you could power it from the 12v rail since you've got an excess of power available there in your setup anyway really, but I'd keep it just relay triggered to battery power.
you can make your valet switch to the ign trigger. you can use a 3 way setup too, 1)switched off & the opus never sees the start signal, it does nothing,2) on it will work normal controlled w ign & 3) override, to keep the pc on even with the ign off... it'll still ultimatley shut the pc down when the battery voltage drops below a threashold, but allow extended run time as an option if you wanted...
I had a thread on this a long time ago, here