That would be my concern as well for the sort of complex diplomacy Boris describes, but I think "joint peace" could work just like "joint war" does now.
That the Designers would have to work on the AI is a given, but given that the AI behavior now reflects some of the behavior proposed, it's not an impossible task: we aren't asking the AI t do anything new, like have a constant strategy or tactics, but some of what the AI is doing now:
For Nos 1 and 2 above, Peace between A and B is weighted as to it Desirability: they share the continent with the AI, their War disrupts some Agenda of the AI, etc. Then it's a question of numbers: we already have numerical scores for Military and Population, so AI have a military stronger than them, I can threaten. AI have not, I use 'Diplomacy'
Nos 3, 4 and 5 above are all based on an-already-defined-in-game Mechanisms: shared Religion, Continent, Government, Policies - we have Agendas depending on them already, so the 'Diplomatic Option' becomes an extension, in some cases, of the Agenda. In other cases, assign a 'weight' to factors: how important is Shared Religion, Continent, Government, adjacent to near-adjacent Borders, etc.
Agree completely that the AI behavior needs to Improve, but This Behavior can be reduced to numbers already in the game in one form or the other, and it seems to me if it can be reduced to numbers it can be programmed - IF the company sees sufficient gain in customer sales and customer satisfaction (resales) to be had from the investment in Programmer Time.