What are some strategies to improve flexion at the lope. Horse is stiff on one side. If I add pressure to encourage better flexion , the horse speeds up or gets frustrated and tries to escape.

Always fall back to trot, then walk or even stand still if necessary. It's common for horse to think they can't accomplish what you're asking and so be sure to simplify things right away when you feel the horse is getting worried or defensive. Be also very clear and only release when the horse has given to the contact and done what you asked. Even if it was way back to a stand still