If they walk like this in a narrow alley, they will block the way.
1. What should I say they are walking if I do not want to use "shoulder by shoulder"?
2. Can I say "others will not be able to 'pass through' if they walk like this in a narrow alley"?
And you could say that others will not be able to get past.
Note that marching is an odd choice. They are walking.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
I was going to suggest 'walking four abreast' but I see Rover and Mike have given something similar.