This issue is so simple. I don't know why people make it so hard. It's called RDS and RD.
Main office:
Server and domain network with RDS installed
Remote Desktop Server
Install AC or any other to RDS and each employee at other office has access concurrently as long as they are licensed to use their own sessions.
Remote office:
Computers with RDP
Internet
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Once you purchase a server for RD, you are all set. Just some configuration. The server will need the role for RDS configured. It is slightly more difficult than that, but there are hundreds of articles on the Internet.
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The cloud
