The “-act#” command line argument doesn’t appear to be working for offline character creation. With legacy D2, you can run this command:
"C:\Program Files (x86)\Diablo II\Game.exe" -act5
…and any new char you create in that session will be in Act 5 at level 33 - with the typical starter items. Adding -act5 to the additional command line arguments for D2R does not do this.