Edition depending some of these are managble even mid game some at low game. It all depends what edition and character/party build you have. Like if you have a 3.5 group with mid level martials for the majority a beholder is less a threat with its antimagic than if you have mostly casters. The context of the table means a lot for these. Als♛o an army of ferrets can take down a tarrasque in some games because at least in 3.5 if you beat the ac you will at least do one damage reguardless of reduction because game balance says you cant just make something that wont take damage.

I think drow may be able to do a wisdom check on them? So drow circle 𝓰of spores druid would be able to but other than that it does seem a bit odd but most adventurers don't want to go to the underdark so I guess it's meh

So about the staff quest. The thieves guild probably doesn't know your the archmage now if you did the quests first because I ne♐ver assumed the mages guild announced that kinda thing. So I find it more a good aspect of not everyone knows what each guild is doing. I'm sure the blades wouldn't be happy knowing we were in the dark brotherhood, let alone becoming a silencer for them later