The problem with the isotope thing is that they have to ASSUME that the isotopes in the area were exactly the same as they were 5000 years ago.
The isotopic composition of the soil is not going to change too dramatically in 5k years. There might be small fluctuations, but they'll be within a margin of error. What really clues you in that they've found the guy's birthplace is that they've found an almost perfect match for the isotopes within walking distance of the iceman's final resting place. If this is not where he came from, the soil would have had to have been A)switched out completely with the area from which he did originate or B)somehow, out of millions of possible outcomes, assumed the composition of the soil from the area from which this fellow came.
Since B is patently unlikely, the only other option would be for someone, on a whim, to truck in several hundred billion tons of dirt to confuse the issue.
Hence, my reply to you.