-Path-gridding in the small island cave makes actors behave all funny. And Volus won't open the door despite him having the key.-NPCs on the isles, when they find the player via an AI package, go to the player and then run away. And they won't escort the player despite the path grids.
How about the player *order* NPC to interact with the world by open the door just like what you can do in Skyrim instead of being scripted (via AI package?).
The NPC opens the door if it's unlocked so I don't know why he doesn't unlock the door. Path-gridding is not really my forte.
Path-gridding is pretty straightforward. What problems are you having with it? Here's the wiki page on the different options. Here's a tutorial that explains how to do it. Here's another one that's easier to read. Skip down to the section on path-gridding. One thing about path grids: don't overdo them. The part in that first tutorial I link in which the author says the nodes are safe points for the NPC to stand on is not true. Yes, the NPC will walk along a path grid in the line you have set up, so if you have a corridor with only a single row of nodes, NPC's won't be able to get by one another. But they can deviate from a path grid slightly, so you don't need a node right on top of a chair marker, for example.
I also noticed you gave up on getting the town crier working. If you're having problems like that, remove all the AI packages and just have him stand there. Go up to him and click on him and see if his greeting works then. If it does, then there is something wrong with your AI packages. If not, then there is something wrong with the greeting. Simplify things while debugging so that you can narrow down the cause.
It's also very important to have a save from before you entered the worldspace or the cell. Once Oblivion has loaded the state of the cell, you will have to wait for the cell to reset before truly judging the behaviour of AI packages and other things that are dependent on the state. At this point, I would do a clean save without VI loaded and do your testing from a save with the mod newly loaded. If your NPC's still won't unlock doors then, then post up a screenshot of your AI packages and the scene with path grids enabled (in the CS). Also show a screenshot of the lock (that has the name of the key) and the NPC's inventory. You can post those in the quests forum if you want to discuss those issues privately.