OP-this may sound a it harsh, but-here goes. You have MANY good ideas here. You have what your VET has said to do (and he has actually SEEN the feet in person. I would suggest the following:
You stop trying to overanalyze this. Step 1- Spend more time in trying to find your horse a dry place to live, than you are questioning what the vet says, that is not productive time, IMO. Without dry feet nothing will work. THis is PRIORITY ONE.
Then, (step 2)PICK ONE of the treatments and go with it. I would suggest it would be best if you go with the vet, or cow stuff, but you need to pick something and stick with it long enough to see it there is a difference. Perhaps a second opinion would make you stop reading on the net and questioning constantly. If you don;t trust your vet get another. It is one thing to look on the net, that is fine, but the constant waffling back and forth....sometimes can even make a vet quit. WHy would he bother if you are always going to do something else (that you read from strangers who have not seen the horse) anyway?
Once you have started a treatment-step 3. FIND A NEW FARRIER. Yours is crap. If I had a farrier who had not mentioned this to me MONTHS ago (if I somehow missed it myself) he would never be allowed to touch my horse again. Period. Perhaps your vet can recommend one who is good at corrective work, which you will need.
God luck getting this under control.
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