who scribbled all night rocking and rolling over lofty incantations which in the yellow morning were stanzas of gibberish,
In a frenzy of inspiration, they wrote poems all night long, only to discover the poems were terrible in the morning.
who cooked rotten animals lung heart feet tail borsht & tortillas dreaming of the pure vegetable kingdom,
They were idealistic vegetarians who ate rotten animals. To be honest, it sounds like they were eating the equivalent of road kill. They had lofty intentions about the animals, but they were so hungry that they ate anything that came their way.
who plunged themselves under meat trucks looking for an egg,
Why would you find an egg under a meat truck? Anyway, these guys were really, really hungry.
who threw their watches off the roof to cast their ballot for Eternity outside of Time, & alarm clocks fell on their heads every day for the next decade,
They threw their watches off of a rooftop in a naïve attempt to escape time, and fate rewards them by sending a cascade of alarm clocks onto their heads. This line has a sense of dark humor in it.
who cut their wrists three times successively unsuccessfully, gave up and were forced to open antique stores where they thought they were growing old and cried,
They tried to kill themselves by cutting their wrists three times but didn't succeed.
The speaker gives us the impression that it wasn't unhappiness that led them to attempt suicide; it was the desire not to grow old.
Needing money, they had to open antique stores, which only made them feel even older and sadder.