You can build a tank archer, but there are tradeoffs to be made when distributing stat and skill points. You'll also either have to invest in Point Blank or keep a melee weapon handy to deal with crowds.
Actually, since the DK's base armour class goes up with his character level, you don't need as high a strength to meet armour requirements.
Sneak can be quite useful for archers as well as mages. I would try it at the default level before deciding if you need to boost it, though. If you use it a lot (for hit and run tactics, etc) you may want to put a couple skill points into it, otherwise the base level should be good enough.
A mage only needs enough strength to equip whatever armour you want to use.
Agility is the primary component of accuracy, though I don't think that is very important if you are only using magic. Agility also helps evasion, though speed is the primary component. If you are not using a conventional weapon as a backup, a mage can probably neglect agility. You might want to start a topic in a BD forum about how people have distributed stat points for mages.
Note that Kaan's mage guide was written for Divine Divinity. Both the stat system and skill system were changed in Beyond Divinity (the second more drastically), though some of the suggestions are still applicable.
