(C) news of these shots that were sent by swift messenger took four days to reach New York City and another eleven days to reach Charleston, South Carolina.
One of the problems with this option is that the modifier "that" can refer to news or shots. Common sense dictates that "that" refer to news but still can be a little ambiguous.
The other problem is that is in passive voice and GMAT prefers active voice.
Option E solves these two problems
(E) news of these shots shots took four days by swift messenger to reach New York City and another eleven days to reach Charleston, South Carolina.
for your second question the problem is that "as many trucks" is not the correct idiom
For instance
there are twice as many women as men
the number of women is twice that of men
these two sentences are correct.
also by themselves is not correct in options C,D and E. The trucks did not come off by themselves, they just came off.
Let me know whether this solves your question.