I realize this might not be an option for kids not homeschooling, but I've had good luck at reducing the number of hurried "dumb" mistakes DS8 makes when I give him fewer, harder problems. Too many problems--even if they're challenging--and he rushes to get done or gets overwhelmed. But if there aren't that many to do, he slows down and focuses better. (Well, most of the time, at least...)
We also have problems with keeping columns straight. Graph paper or requiring as part of the assignment that he draw in the lines for the columns on each problem helps.
This doesn't address the issue of reading comprehension, obviously, but maybe doing something analogous with reading could work? Give a shorter, harder passage to read? The "columns" could be comprehension questions given at the start of the reading assignment, maybe?
Trying to help, even if I fail...
