@Olivier_Leroux
I know you are aware of my module, but it may interest you to know that it is designed to play in coop mode as well, and I put a lot of effort into ensuring setting up and playing coop is easier (stable) as can be for new players. Basically, as long as the host starts the game each session, then it’s easy enough for up to three other players to join the game, and even allows for an extra two PCs to join the party. It comes with a guide on how to startup and play coop.
My wife is currently running through a hosted session (by herself) just to test the coop setup aspects again, and apart from a couple of minor issues (which may also affect SP) she has not encountered any difficulties. Note, as normal, I am patching the campaign regularly anyway, and so any important updates will also be covered in the coming days and weeks, if required. And, if you did want to try it, I will be here to help support your MP coop sessions as well if it is ever needed.
And, so you know … You do NOT require Hamachi, but instead you can open ports on your router to allow direct connection, which is the best way to a play coop session. i.e. The HOST starts the game multi-player, and then the client connects via Direct Connect to your IP address. As I say, my own module has been tested as working/connecting fine and has been designed to work with multiple players as well as SP.
If you are interested, let me know if you try it, and I’ll try to be around as and when for any further guidance to help you and your friend(s) play. Only day I am not around is a Sunday. Thanks.
EDIT: A download link to some instructions I wrote a long while back, which I think show some help. (Ignore DM part, as that is no longer supported.) While quite old, the main gist of how to setup a coop game is still correct.
Basically, you can employ the SAME steps when starting the official game too. (Although, the OC does not come with the same warnings and help as my own does on startup, so just be prepared to ensure the host is properly up and running prior any clients joining, as that is the main point to consider.)