I think the trick is perhaps to have limited responsibility for the faux-NPC's actions but also have limited benefit. For example, if would be bad if online players could farm faux-NPCs for loot or XP. It would also be bad if the faux-NPC got itself killed many times or used all your potions. Maybe they never use expendable goods, and there's no/little/capped benefit to players killing them? Similarly no/little/capped downside if they get killed. And you only get a little XP from their actions, e.g. 30% normal. But for being a merchant etc, sure. They sell what you allow to who you allow.

1) speed is not an issue, you can do it in a seperate thread/computer = 0 performance cost.
2) fact : computers are better in combat than players. A computer will interrupt your strong spells at (0.001sec) before spell finishes, thus stunning you for an 2 seconds than a normal human can achieve. A computer can execute a combo sequence flawlessly and there is nothing you can do to prevent it. A computer ai can dodge ALL your skillshots, no matter what you do. A computer can have better skillshot accuracy that you once it adapts to the enemy personality. Generally speaking perfectly designed ai vs player, ai wins everytime regardless of player actions.
3) allowing players to automate parts of the game is a valid strategy, if they find something "interesting" they play it, else they let the bot do its job.

I'm trying to solve a similar problem - asynchronous interaction for a browser rpg game. We have the resource gathering, crafting and trading systems similar to what you describe (somewhat) done, but it this approach breaks at combat system and keeping a faux NPC safe.

[color=#ff0000]Depends on death penalties and possible actions by players and npcs. Depends on if and what pvp system you use.

The point is that non-combat actions can be queued and done by some mindless AI, but setting tactics for fighting and relying on AI to execute them puts your character's life at greater risk than doing the fighting yourself. I fear that it'll be easy for a logged-in player to employ superior tactics against faux NPCs and lead to a situation where you need to be online at all times instead of having a life - fNPCs can be easy prey for players, so you'd want to minimize the time you need to rely on AI to keep you alive.

[color=#ff0000]I do not use PvP for my game for various reasons so I sidestep this issue. Generally NPCs would want to band together against players, would want to have safe zones, and you would want to minimize death penalties. Full loot ffa pvp does not make players happy in most cases, so choose your target audience and use the pvp mechanics they are likely to want.

You gave an example of faux NPC defending a town from a raid. That means you need to make sure the fNPC can stay close to logged-in players, knows when and where to flee (when it won't be possible to defend the settlement), doesn't get surrounded by an overwhelming enemy force and so on - seems to me that there's a lot of things that can go wrong, cause a character's death, loss of items/experience and piss off a player.

[color=#ff0000]I suspect the player would be more upset to lose the town they built. All buildings in my game are player made. The world is huge so players can set 2 bind points and likely the backup will be away from monsters for respawn. NPCs will run on a separate code for settlement defense. If another player tags them for control they will follow the players instructions instead of their other code. You can set actions, like: man the ballista, guard the gate, spells to cast if you have any and so forth. Imagine the AI controls in an RTS where range tries to run if melee enemy is too close and so forth.

There's also a problem of making decisions that belong to the player, like 'should I use the Awesome Potion Of Healing That Heals Instantly But Is Rare or should I use something weaker and try to dodge attacks for now'. Of course it can be done with preset tactics, but this can lead to players essentially scripting their own AI to keep their characters alive.

[color=#ff0000]Players who focus too much on their own character will lose out in the over all war against the environment. There are no insta heals for players or ceatures. At best Morrowind style hot heals, even with potions. All items in the game are player crafted. If you got the potion once you can get it again. Likely consumables won't be super expensive anyways. Its not like they stack. [color=#ff0000]Something helpful might be to imagine the characters as the heros in Majesty. Except instead of rehiring player characters respawn. And the main goal is to protect the buildings that get you money and items and defenses and stuff. Its basically a giant RTS except instead of units, players.

About other players controlling fNPC: I don't know what the fNPC to logged-in players ratio would be, but i suspect more than 3:1 (25% of players logged in at all times is 6h of playing a day, and they won't all be guarding fNPCs). Therefore in any combat situation every player would find himself responsible for keeping at least three AI-controlled (possibly suicidal) characters alive and giving them orders while trying to survive himself.

[color=#ff0000]Players choose whether or not to take control. Assuming the offline player has put them on the permission list. FNPCs will function based on player selected imperatives. So you could set them to fight with typical anti siege tactics. In any case players should expect to lose many settlements over the course of the game.

It seems that players will want to leave their characters in places where no significant fighting will take place and combat orders would be 'run like hell'.

[color=#ff0000]Fat chance. There are no safe zones except perhaps the world gate and the world is huge. A single FNPC would probably not be able to make it across the wilds to the nearest settlement.

Anyway, it seems like you have the non-combat stuff planned, did you plan more details on combat?

[color=#ff0000]I have maybe 10000-20000 words in various threads here and at mmorpg.com. I also have a website, its not very well made, as it is not my main priority and I also have documents describing all the features and interactions for economic, social, magical, political and combat related functions.