By saying that infiltrating Teron palace is easier going than infiltrating the slums is not a particularly reasonable proposition. That would be the equivalent of saying that sneaking up on Trump/Xi/Putin/The Queen of England in their official residence, surrounded by bored out of their mind guards, is easier than sneaking through the former walled city of Hong Kong/chicago gangland/a favela. I doubt that anybody irl is going to agree with that statement.
If an assassin can sneak right up to an actual head of state without issue, claiming that gang bangers prevent the assassin from using stealth instead of the trained soldiers or bodyguards is again not particularly reasonable.
You're comparing current time, real-life situation to a post-apocalyptic setting where literally everyone is out to stab their own neighbors to fulfill their selfish desires. Have you paid attention to the lore and narrative of the game? It's a world where civilization is in a state of indefinite stagnation. The Great Houses are losing their powers, while the other factions are growing from either being left alone (The Imperial Guards) or leeching off the Great Houses's power (The Thief, Assassin, and Merchant's Guild). Introduction to arriving in Maadoran describes it perfectly:
It shouldn't be hard to comprehend how the Forty Thieves Guild are able to protect their territory so tightly, not to mention it's completely unfamiliar territory too huge for your character. I mean, have you tried playing as a Praetor, telling Serenas (Gaelius's nephew) to try and torch the Slums? Instead of the Aurelian soldiers taking over, likely what you'll think will happen if choose that option, it instead triggers whole chaos that swallows the entire city of Maadoran. That's how ferocious the Forty Thieves guild of Maadoran, is.
The point about failing the impersonate check on purpose goes back to the issue of legitimate C&C. If I remember correctly, the game railroads you into the impersonate check. Choices are only granted once you are in front of the patrol. If you cannot pass the check, you eat a penalty either in the form of losing gold or starting a fight without any armor on.
How is this an issue of legitimate C&C? The game straight up recommended you to invest in Impersonate, you decided not to, so now you face the consequence of having to use whatever means you actually have in your arsenal from compensating your lack of investment in Impersonate, either by paying a beggar to lead the way or just face them with pure combat skills.
The assassin, in character, knowing that he has no skill in impersonate, is forced into a decision which he would not have ever made. This is an assassin, that can sneak up on a head of state. Yet the stealth option is not present at the beginning of the quest. This is an assassin that can go toe to toe with imperial guards. Yet the option to screw the disguise and go in loaded for bear is not there. Yes the game presents choices, but in cases like this (which are a minority I admit) your character's skills and history suddenly do not matter.
Except, again, the Slums is a completely alien territory belonging to another guild that's may be as big or even bigger than the Assassin's guild. While the lack of stealth option could be from the root cause of the whole game's system not allowing you to manually activate stealth, the narrative of this sequence make sense. If you try to stealth your way through the Slums, an unknown territory, of which it's choke full of the Forty Thieves guild members likely as skilled as you or even better in subterfuge, you'd likely ended up getting stabbed in the back instead, or have to face another combat encounter anyway. Hence why the game's presented solution for this sequence in the first place is to check for your Impersonate skill, because it's the only way to fool the thieves, who, again, are likely as skilled or even better than your character at subterfuge.
The issue about "knowing" to invest in impersonate just proves the point about veering from the path. The game has the idea that a proper assassin is skilled in impersonate. Choosing not to invest in that skill and instead in something else, ie veering from the path, forces the player to brute force their way through rather than deploying their skills in a useful manner.
You're implying playing a fully combat focused Assassin is not possible, here. And therefore, trying to play a full combat Assassin means not deploying their skills in a useful manner. It's akin to saying how, during Merchant playthrough, you can't go and assassinate that visiting merchant in the inn by yourself, and instead having to persuade and pay an assassin.
I'm not saying AoD's system is perfect, but saying things like "not allowing you to veer off from the path" and "deploying their skills in a 'useful' manner" just reeks of you wanting to do anything you want, regardless of whether or not it makes sense from the game's narrative standpoint, without meaningful consequences biting you back in the ass.