1). Not sure what you mean by this, can you include some "sho route" details indicating the problem. The standby PIX shouldn't be participating in OSPF at all until it becomes the active one.
2). Just as a router does, it'll prefer the route with the least cost. If they both have the same cost and are the same route type, then because they're on different interfaces it'll use the first one that it learns. If that one goes away it'll use the second one out the other interface. If the first one comes back, it'll continue to use the second one until that goes away.
Additionally, type 1 external routes are always preferred over type 2 external routes, just like a router.
3). Just the same as you would with a router:
router ospf 100
redist ospf 200 subnets
router ospf 200
redist ospf 100 subnets
You should of course set the metric-type and metric, but you get the idea.
Always remember that the PIX OSPF code is taken straight out of IOS, so whatever you do in a router is the same as what you do with the PIX. Same commands, everything (usually).