In the event of a chain split, does the DAR ensure that competing chains stay relatively the same length regardless of relative hash power?
Yes. In a 50/50 split and no DAA change on either side, it would take ~2x as much time for each branch to mine their next block, and then the diff would adjust downwards. Then 10m blks
"Longest chain wins" only applies to miners extending the same chain.. if BCH splits into 2 incompatible chains then they both would become the longest chain of their own consensus