1. AFAIK computers work on sampling the depth every 'x' seconds. They use the Haldane equation, similar to Schreiner but meant for all the time at a constant depth.
2. You can use whatever units you like provided you keep them the same throughout e.g. if you use MSW then you have to convert gas partial pressures to MSW too. Same for bar, psi, fsw, whatever.
3. There is a worked example on
this site
Kevin Gurr must be making a fortune, I don't know why. A trimix computer is no more complex than an air one. There is no way I'd pay that. Anyone who can write PICs and knows electronics could produce one (I'd happily sell my ready written C source code! Hint! Hint!). I toyed with the idea of making XS commercial, it even got reviewed in Scuba World and a French mag years ago, but that wasn't the point of the project, in fact it was the complete opposite. Besides it was fun to piss off the makers of Proplanner.
Cheers,
Stuart