Ouch!!! With the plantar fascists, what helped me was slowing rolling my foot over a cold can or a golf ball. Hurts like buggery but stretches them out .
I also lost some weight as the extra pressure was hurting - may not apply to you!
Re rhe pronation, there are trainers out there designed for that. I over pronate and get trainers from new balance and asics which really help. X
I have this too! I have flat feet and got it when pregnant. I still have a weakness there now. New Balance trainers are great support as is rolling a ball with the foot as mentioned above. I was told told not to wear flip flops at all and to do calf raises and stretches. Good luck!