It is highly unlikely that other routes will be used to reduce any delays at Dover. It has been a good few years since any services ran from Folkestone or Ramsgate. Even if they resurrected that half baked idea of running ferries from Ramsgate, you would still have the problem of the vehicles entering Kent.

Ports further west from Dover have the problem with the length of sailing time. They could in theory put on extra ferries but that will depend on costs etc.

When I first started in the business, ferries from Dover ran to Calais, Boulogne, Dunkerque, Zeebrugge, Ostend. Nowadays they just run to Calais and Dunkerque.