Hi Blackrain,
Please note, as per Sam’s comment in another thread that the update that is currently rolling out on Playstation is one that was scheduled before the co-op version was released. This is not the fix for the interactions bug, this fix will be in a new update and is still in progress.
Edit: I have made a note of requiring a restart of the game to reload a save as a seperate report for the team. Also, you are correct that Multiplayer and Single player saves are different save types. You cannot convert single player saves into multiplayer games, however if you create multiplayer private as you have already done, you can play it as single player with the option to invite someone at a later stage.