I've just found and read all of this, and all I can say is what awful response from Kep. Based on their response alone I would be put off buying one. OP I am glad you are ok, and sorry that you didn't get far in your investigations.
There is a thread on Facebook currently with a vid of a...
So long as the box was the original box the bute came in, then each sachet of bute within the box will contain the same serial number and this will match the serial number written externally on the box. Both should also have the same exp.dates on them.
Your vet should also have a record of the...
Try Sarah Steel ( Yorkshire Saddlery Repairs) got a Facebook page, she has a good range of second hand saddles. I've used her and had several people recommend her.
Like Ester said Osphos and Tildren are both working to extend the life of the joint, not fuse it. You could actually fuse the joint by injecting ethyl/alcohol, this has quite a high success rate.
Also the steroid injection may well help the joint (the risk of laminitis is there, but as far...