Your biggest problem is the number of desired interfaces. As far as I know, the maximum number of interfaces on the 515, logical and physical, is 10.
I have a bunch of PIX boxes using trunks, and I have a bunch of PIX boxes in a FO bundle. I have never tried to both trunk and failover at the same time.
Without putting the scenario in the lab, my best guess is that should the physical interface fail, that failover would happen in a normal fashion. I suspect that should you somehow lose an individual VLAN, that the PIX would not failover.
I would also be concerned about the general architecture of such a scenario. Your essentialy creating the stereotypical "router on a stick". If your traffic patterns are for the most part from outside to protected interfaces, your probably ok, however if you have a great deal of traffic transversing between internal subnets, your creating a great deal of innefficiency, albeit in a secure manner :-)
Given that you have a 515, the FWSM blade and 6500 is probably economicaly not doable. I would however, look at attempting to use layer 3 switching behind the firewall, where my security policy would allow it, or migrating similar systems to interfaces with appropriate security levels to reduce the number of interfaces.
