As Kael said this possibility would be far away from realism. In real world contracts it was specified accurately where you can lead your army and where you can't (for example you may move the line till river x and if you do more you will be the contract is broken and if you don't retreat imidiately you're at war). Normally an open border to an enemy should also include who you hit with that, because the AI gives them to everyone even if they are only cautious, regardless if the person with which you have an open-border-contract wants to attack your best friend. An openborder treaty to an enemy is a declaration of war.
If you're realistic there have to be neutrality pacts in case of war, if you're realistic cultural borders are nonsense (noone accepts a city running over to the enemy without declaring war or at least a certain treaty or a democratic votum like it happened in Germany after WW II), borders are made by negotiations (which are of course influenced by the question of cultural influence), if you're realistic there have to be hard negotiations with everyone who has a certain interest in that area (so a neutral civ won't block ALL your conquered territory by cultural influence). If you're realistic diplomacy is much more important than it is in all parts of civ (except Rhys' and Fall. The first diplomacy system that persuades me. Diplomacy - and bribery - really mattered). You cannot simulate accurately a so complex system as the real world diplomacy, civ will always be a game.
That said your first idea is really good, but your second is far away from realism. I am anyway pleading to separate trade treaties and open-border-treaty, because the first happen quite a lot (even between partners, which don't know each other quite good), but the second needs a high level of trust.