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The new VPN status view available now does show 3rd party up/down VPN status as well as latency and usage.

You should be able to see currently connected VPN users through the regular monitor > client view.  You just need to filter by the VPN subnet.
Good idea.  I would add CPU and RAM use as well.
Yes, I discovered that too and it is documented here that all MX systems within an organization must use the same license. https://docs.meraki.com/display/MX/Licensing