Just how narrow is that 'narrow alley' your character is walking down? Is it three meters wide? One? Five?
As for guarding crowded in crowded spaces, I don't think that that'd be a good metric to base things off. So much of the game has the crowded flag now that it's kind of.. not really useful. Places like the hallway for apartments with only four apartments present are crowded, when you're only reasonably going to have like 1-8 people coming and going to their apartment at any given time. I hope that example makes sense.
But it shouldn't be hard to code difficulty multipliers on specific exit types. Guarding a door? Easy. Guarding the street or a Badlands tile? Harder.
If they try to flee or charge in your direction, it just doesn't make sense that you'd be utterly impotent to obstruct it.
Another issue I see is that the type of room itself doesn't say anything about the exit itself. Street and outdoor rooms have small exits that could be guarded easily. There are many large open areas with doors or passages to smaller spaces that wouldn't really make sense to made arbitrarily harder to protect.
I agree that it could be an odds thing but I'd put the odds onto the chances of them being able to shove you away from the exit in the first place. If you try and guard Sinn then there's a real good chance I can just cruise on by as if you weren't even there. No need for me to use special commands.
I suggest this as, if they already shove a character away and disrupt their movement, 75% of what guard is good for is already done did. In my opinion.
