[@Dead Cruiser]

The main reason for the ban in forbidden magics is that Aloysius himself is not keen on them. (Though he may be hypocritical there.) He us trying to win over the hearts and minds of the zmsgi to his cause. Most if the forbidden magics are considered taboo, so if he allowed theur use in the DS it woukd only strengthen the popular opinion of the Magi to have them destroyed.

Forbidden msfuc includes Necromancy, Blood Magic, Poession Magic and chronomagic.Rune Magic us a little but more complicated. Rune magic involves drawing a symbol upon a surface using magic infused energy, usually drawn frkn the casters soul. These runes, depending on what was drawn can have varying effects, from harmless to outright lethal. Ontop if this, Runes tend to degrade the soul if the user, shortening theur lifespan with each use. Given the lack of Magi alive anyway, losing done to Rubemsgic could be problematic. 

Therefore Rune Magic is mostly taboo. The only really exception is the use of defensive runes to fortify locations etc. This is down to the fact that defensive runes are less soul intensive and the would can recover from them. Offensive runes deal alot damage to the soul and in some cases are even irreparable. 

As for your suggestion with ghosts and dead interaction: The walking dead is a thing that doesn't exist. That kind of necromancy hasnt been seen in a long time and Salem and Al are both in agreement the would want to keep it that way. As for ghosts it is indeed possible for the soul to stay behind after death. Normally they pass onto the afterlife completely of their own accord. Occasionally they have to be pushed. But someone could interact with them. (I don't want to say anything else as we start getting into spoiler region)

I hope that has helped you somewhat.