For your question1: you can have multiple terminals with ssh. What you have to do is to open multiple terminals on your own computer, and from each, start an ssh connection.
Question2: Yes exactly.
To start with this Goal message, I recommend you do the following in this order:
1. Use the Niryo One Python API, especially the “move” commands.
2. Once you know how to use it, check out the code inside the Python API, to understand how the move_joint command works with the Action client
3. And now you can start your own node.
There is an alternative method. If you want to directly control the arm movement, instead of going through the ROS Action, you can bypass that and use the Moveit! MoveGroup functionality. Check out this part of the code where we use the MoveGroup, and also check this MoveGroupCommander tutorial to know more about it.