1. WSFC/MSCS for 2012 (minimally) supports "64" nodes per cluster, but vSphere only supports "5" WSFC/MSCS nodes per cluster?
Yes, because "5" nodes are tested by VMware that doesn't mean you can't extend to "64" nodes. If you go beyond 5 node cluster and if you run across any issue VMware will not be responsible.
2. Assumption is for a 5-node WSFC/MSCS cluster running within a 10-node ESXi cluster, that any WSFC/MSCS node can run on any of the 10 ESXi nodes, just not simultaneously (e.g. anti-affinity rules must be in place).
Yes, you can run on any 10 nodes, but you need to be few check before you create anti-affinity rules.
1) Make sure you have access to shared disk across all 10 nodes.
2) You need to create VM-VM and VM-Host anti-affinity rules.
3) All hosts are from same vendor.
I hope I answered you queries.
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