I am wondering if there is a Command-Line argument that would launch D3 into Adventure Mode and always drop me into the Act 1 map instead of a random act every time.
No.
I would like to argue that command lines should be square.
Also, they are not random. It cycles through them and changes acts on the hour, every hour.
-actX: (replacing X with the act # to start such as -act5:) used to work… a long time back, in D2 days. But, not any more.