His chronology is subject to some argument since it is difficult to place the Battle of Badon after 499 (most likely around 495) and thus also difficult to place Arthur's final termination much later than 515.
On the other hand, Merlin may have been a later Arthurian figure in Arthur's second period and have then survivied Arthur's death by some extended period. I haven't read Ms. Goodrich's books in some time and have forgotten where she put him on the time line.
And the fact that these people were based north of the wall does not preclude them from having significant action in England where the enemy (the Saxon's) were located. Most of the serious students view Arthur as a calvery military officer who was able to take the military action to the enemy everywhere on the island that he found them.
hmmmmm ping