Chronic plantar fasciiitis plus high arches--doctor put foot in walking cast 7 days ago and pain has been much worse since then--is cast a mistake?

Pain: Anything that causes an increase in pain should be reaccessed. You should look into exercises for plantar fasciitis. When this is over, it has a high likely hood of returning. There are exercises that will limit or hopefully eliminate the PF.

Not a mistake but...: The main thing needed to alleviate symptoms of plantar fasciitis is good arch support. There are many options, including doctor-customized shoe insoles (orthotics) which can run up to $300/pair or ones you can buy online and customize on your own at home for far cheaper. The latter is probably the better choice. As a runner, I suffer from PF too, and since using orthotics my PF pain is gone.