Actually I don't know much about the Microsoft Graph, but I could give you some hints on my part(O365 & Teams)
Are you using the beta version Graph?
Because I didn't see the Meeting Created Count attribute for getEmailActivityUserDetail from the Graph 1.0 document:
- Report Refresh Date
- User Principal Name
- Display Name
- Is Deleted
- Deleted Date
- Last Activity Date
- Send Count
- Receive Count
- Read Count
- Assigned Products
- Report Period
And as for your second question, I think it's a YES. Because the Teams meetings created on Outlook clients should be added to Teasm Calendar soon.
About Microsoft Graph 1.0 and Beta version: https://learn.microsoft.com/en-us/graph/api/overview?view=graph-rest-1.0#other-api-versions
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