If it would be couple of weeks, Larian would be allready teasing us ... they usualy anounce PFH coming at least 2 weeks in advance.

So i would dare to say that right now, it would be safe to presume that patch 9 is at least a Month away.
Any advice on taking on Minthara? Especially how to destroy the warning drum without alerting suspicion.
Nope ...
The only way is to fake your intention to join her assault and betray her at the gate, where the fight is quite easy ... but its quite easy even where she is ...
As for the drum ... im affraid there is no way to destroy it without her noticing ... maybe if you would cast there Silence and/or Fog mist? Not sure really tho ...
I never cared about those drums to be honest, since no Goblin ever managed to get close to them.

Also, can someone remind me how to access the spellbook for magic users?
If you open your character, in top left corner you see small button looking like open book, thats it.

Are there no "area zones" that remain neutral? For example, if I kill everyone outside, are everyone inside automatically hostile? What if I only kill the sleeping Gobbos upstairs outside, are the revelers downstairs also all hostile?
Not if you dont get caught ...
If someone notices you, they yell alert, if not, you are free to assassinate as much goblins as you can.
