When making a site accessible by allowing keyboard navigation is it OK to use the left and right arrow keys to move between columns (in addition to the tab key which traverses all of the column's content before jumping to the next) or should these arrow keys remain free for horizontal scrolling, much like the up/down arrow keys are exclusive to the vertical scrolling?
It's definitely OK to use the left-right arrow keys for navigation if the page cannot scroll horizontally. If the page can scroll horizontally, the conventional behavior of arrow keys is to scroll the page.
The answer depends on whether your website scrolls horizontally at standard screen sizes, or if it is responsive. You don't want to override default behavior if that behavior is needed to access your website. If not, then go for it. Adding key shortcuts is generally a good thing.
However, keep in mind that with non-standard key shortcuts, users won't realize that they exist unless they are told. So, make sure to display some sort of tool tip or message that indicates the arrow keys can be used as a shortcut.