commonfish, I can so relate to a vast majority of what you've experienced and have done to try to resolve.
My farrier, though, did not agree that removing the shoe would be more advantageous at this time.
We have been using FF 2X and up to the last month, our farrier has said Chuck's feet (soles and hoof wall) have never looked so good.
Our vet has assured us his feed & forage is what he needs.
We have also been dealing with thrush which is now back under control... You wouldn't believe the routine I have to try to manage that... ;-P
Long/short, whomever talked about the wet potentially being a big part of this issue is right on. For us, it isn't just the actual rainfall. It is also how much dew there is on the grass and for how long each day. Like our vet says, 'welcome to Florida'. We moved here just under two years ago and throughout this time our horse has had to adjust to quite a bit. Luckily, this year we have ridden more than we ever have and our boy seems to thrive on that 'work'.
So, Chuck is back in shoes and we hope to be able to keep this set on.