Sorry man, this board is pretty dead these days. Local folks have moved on to non-Subarus or have taken their car club discussions to RenoSCCA.org or Facebook.
Anyway, my first guess was your first guess: head gasket. But if the coolant is really clean, it seems unlikely. You might want to do one of those hydrocarbons tests on the coolant.
Otherwise, could you have lost a ringland? I didn't think that was common on the non-turbo cars. But I guess it's possible to burn a ton of oil with little smoke in the exhaust... at least until the cats burn out.
The oil being clear of metal is a good sign, you may not need to rebuild it. But the fact the motor was lugging after the oil light came on sounds pretty bad. So who knows. If you're DIY enough to do the head gaskets, that's what I'd do. If it doesn't work, you're just out the relatively low cost of parts plus a weekend of your time. Plus if the heads are off, you'll get a good look at the ring lands.
On the other hand, if you're handy enough to do a motor swap, a used 2.5i motor can't be that pricey. And you'll probably be good for enough time to drive it a bit or trade it in on something newer. Then again, that plan might work after just doing the head gaskets.