If there is new stuff then probably a new ticket for that (link it to this one tho)

Just double checking, with "stuff" you also mean bugs, feedback and or answering questions you posted in your latest update?

No longer triggers on non-existent sections but still triggers for non-existent attributes. i.e:
"BogusSection" set3DENMissionAttribute ["BogusAttribute", ""] no EH trigger.
"Multiplayer" set3DENMissionAttribute ["BogusAttribute", ""] EH triggers.

In general naming of script commands for 3DEN is already inconsistent, like these are all entity specific:

• add3DENConnection/remove3DENConnection/get3DENConnections
• clear3DENInventory/save3DENInventory

Ah I didn't think of that, ofc if attribute changed handles undo, you can handle everything. I have not tested if that works, please confirm if thats working.

After doing some testing on the current profiling branch (2.14.151108) here are some things I found

Chat is controlled by script, showChat command. Eden script changes are too complex and hardcoding it in sounds meh.
I should probably hardcode it. Postponed to later maybe.

OnBeforeMissionPreview

Now, for the first case, we can trigger the eventhandler (and let you abort it) before we enter MP mode.
That is easier to implement. But that means if you block it, the player never starts up the server host and cannot switch to MP mode. But I think that's what you'd want anyway?

Another nice to have would be:

Also you seem sorta lost/not to have read the ticket. This ticket is specifically a list of functionality that I'm requesting in the context of what I believe is missing in order to make a multiplayer 3DEN mod.

That event isn't required, you can achieve the same effect by combing the init and onMissionPreviewEnd events.
Note that while init does pass the display as param, onMissionPreviewEnd doesn't which is easily fixed by running findDisplay 313 (313 is 3DEN display).

After some more testing it seems you don't have to paste right after deleting, you can place as many new objects after the delete as you want. This happens to units copied before a delete happened, then it's group will be set to [0,0,0] after Undo -> Redo

Thanks for fixing this.