steam-user doesn't provide any way to specify your own sessionExternalIP value, but there's nothing stopping you from spoofing that field to be whatever you want, either. It's client-controlled and isn't authenticated by Steam as far as I'm aware. You could try changing the value directly in the steam-user code here and see what happens.
Yes, the internal IP is determined from the private IP specified when you connected to Steam. By default, steam-user sends 0 unless you change the logonID value in the logOn method. You need to encode your desired internal IP as a 32-bit int, then xor it with 0xBAADF00D.
For example, 192.168.1.2 encodes to 3232235778, then xor that by doing 3232235778 ^ 0xBAADF00D and you get 2158493696, which is what you should use for your logonID.
Yes, ownership tickets are cached in userdata/your_account_id/config/localconfig.vdf under apptickets.
Not as far as I'm aware
You can use NetHook for that. When you inject it, you'll need to provide the filename of the game process you want to inject into.
