Here are some mistakes you made:
"Не огромные ямы в центре рахитного урбан, а детали дизайна стиля архитектурного, не разгон манифестации, а нуар балет."
"WE AREN'T giant holes in the center of the rickety city, but the details of ARCHITECTURAL STYLE'S DESIGN, aren't manifestations of critique, but a ballet noir."
"А" as you figured right, can also mean "but". Here it has this meaning in both cases. "Детали дизайна стиля архитектурного": there's no comma here, so it's not an enumeration of what "we're details of". Though by the intonation you can't really tell if there's really a comma here, so your variant might be correct too.
"The blood clot will come off in the gray head"
There's no mistake here, I rather would like to explain your confusion. You confused the verb "отрывать" (to tear off) for the verb "отОрватьСЯ" (to come off).
"Серый человек ложится в серый гроб"
"The gray man (IS LAYING DOWN/LAYS DOWN) inTO a gray coffin."
"Нахмурит бровь, кинет тяжелый взгляд, скулы бетонные сведя, неизвестный солдат, мазню наивную растопчет кирзачом гранитным"
"The unknown soldier will furrow his brow, give a grave look, taking down the concrete cheekbones, he will trample down the graffiti with gray kirza"
In the Russian language, the subject doesn't have to be at the beginning of the sentence. Watch the quotation marks and have in mind that sometimes those who upload texts here don't watch them close enough. For instance: I've had to place a comma after "кинет тяжелый взгляд" because of the participial turnover in the next line.
"Плесни маляры на асфальт, холодным солнцем согретый, и напиши рядом с номером "соль и меф" трафаретом"
"Splash some paint on the asphalt, warmed by the cold sun, and make a stencil inscription "salt and mephedrone" nearby"
"МалЯра". As far as I could guess from the stress and the ending here, it's jargon for "paint", though I've never heard such a jargon before.
Best of luck)
halvframkalt
Again, I'm absolutely open to any input if I have made any mistakes in interpretation. There were several lines which didn't quite come together in my head; I just put together what I could.