According to cisco you should be able to have a PVST+ switch act as the CIST ROOT if ALL vlans on the PVST+ switch have the lowest priority.
When I lab such a scenario I get PVST Simulation Inconsistent errors on my MST switches.
I also tried doing this with the common vlans between my PVST+ and MST switches, all assigned to the IST 0, but still with the same problems.
Could someone explain why this is happening, and if what I am trying to accomplish is even possible?
Yes it is possible, but not with how you have it configured. I could try to explain it but for the sake of brain power, take a look at this excerpt from Petr Lapukhov's blog where he talks about PVST+/MSTP interoperability:
"1) MSTP domain (either a single region or multiple regions) contains the root bridge for ALL VLANs. This is only true if CIST Root BID is better than any PVST+ STP root BID. This is the preferred design, for you can manipulate uplink costs on the PVST+ side and obtain optimal traffic engineering results.
2) PVST+ contains the root bridges for ALL VLANs, including VLAN1, which maps to CST of STP. This is only true is all PVST+ root bridges BIDs for all VLANs are better than CIST Root BID. This is not the preferred design, since all MSTIs map to CIST on the border link, and you cannot load-balance the MSTIs as the enter the PVST+ domain.
Cisco implementation does not support the second option. MSTP domain should contain the bridge with the best BID, to ensure that the CIST Root is also the root for all PVST+ trees. If any other case, MSTP border switch will complain and place the ports that receive superior BPDUs from PVST+ region in root-inconsistent state. To fix this issue, ensure that PVST+ domain does not have any bridges with BIDs better than the CIST Root Bridge ID."