Hmmm. Not heard this one. That said, I've read quite a lot of Tolkien biographic material, and I'm not aware that he ever visited southern Poland before writing the Lord of the Rings. He wasn't particularly well travelled before he became famous, and as far as I know he had only visited South Africa (where he was born and left as a very young child), Switzerland (walking holiday) and France (WW1) before the bulk of the LOTR writing occurred. After 1939 a trip to Wieliczka would have involved him entering a WW2 war zone or crossing the Iron Curtain, something I'm certain he didn't do before LOTR was published in 1955.
Again, I don't think so. I just re-read the relevant chapter in Humphrey Carpenter's biography of Tolkien. The 1911 Switzerland trip (when he was aged 19) was just that; and the group of 12 returned to England as soon as the walking was done.
But there is AFAIK no record of JRRT himself visiting there and Poland would be a very untypical vacation destination of a British guy in the interwar period.