Ergative + Accusative?

I've heard several times that some languages (the example
was Dyirbal) use a mixed system in which sentences with
1st and 2nd person subjects follow the ergative/absolutive
system, while sentences with 3rd person subjects use
nominative/accusative.

Assuming this is true exactly like that, is it likely that
the erg/abs sentences have an antipassive but no passive,
while the reverse is true for nom/acc sentences? The
mixed system looks already weird enough to me...

In any case, is it likely that a system like this could
lose case endings, case being distinguished by word order
alone? In this scenario, I'm planning to keep the case
endings (which would be clitics from the start) only to
mark emphasis (on topic and/or focus) -- therefore these
marks would be different for each kind of sentence. Again,
is this *too* farfetched?

This is all for G'amah, the orc/troll language, but I'd
like to keep it looking relatively human. I'm planning on
VSO order, with accusative objects marked with a proclitic
maybe, but only them.


--Pablo Flores
  http://www.geocities.com/pablo-david/index.html
  http://www.geocities.com/pablo-david/draseleq.html