Well. I try not to railroad my players too much, but if you were intending to follow Tro'Leeras last suggestion to wait until the end of the week when you have developed fake id's before infiltrating the city, that would take pressure off me. It would stress me out because i'd be torn between not changing things and letting you guys walk in to get captured or softening things and running the risk of ruining immersion. I want to make infiltration of the inner ring as scary as the sand goblins or worse.
Context maybe? It's commonplace to see heavily guarded FAS inquisitors surrounded by 4+ armed guards in full plate. These inquisitors are all outfitted with gems of true seeing, which is why any disguises used must be non-magical. They have other magical items too, but this varies per inquisitor. They capture you, they dungeon you, torture you, brand you, then send you to the arena, where you fight until you die.
The reason the thieves guild is so competant in dealing with and evading FAS is that it is rumored to have access to have powerful magics itself (at least, anti-divination stuff), but at the same time, makes use of good old fashioned bribes with guards, false id's, conventional disguises, deception and all other things a thief is good at. Just be aware that this city is very dense and is crammed full of a variety of dangers. Sometimes you MAY run into things above your level. I will give you ample warning. Other times you might bump into potemtially benevolent beings such as the dryad you want to meet And may gain an ally. I could go on, but you get what i'm saying. You don't ALWAYS have to follow the flow of the narrative, especially if it's against your character's beliefs, but you can.