Doesn't the BR have the option of using either portion of the base? I thought the running lane essentially becomes a double-wide.
Yes on both counts.
When the fielder uses the colored base (for example, when pulled to the colored base by an errant throw or when taking a throw from foul ground on the first base side of the foul line) the batter-runner may use either base. In these cases the running lane is three feet wide on both sides of the foul line.
(Which I'm sure is what Comp meant.)