Hello mate,
I have ran 255 on my rears on 9.5 j rims for along time. Before i had the racing harts i had 9.5 j volks with the same size tyre and never had problems. I have taken my car on track, drag strip and 170mph+ without any issues. The reason i stuck with this size was 265 would scrape on my car.
I think your tyre fitter is not very experienced/or trying to sell u some new tyres, as first he said the alloy was leaking itself when he should have easily found the bead to be broken due to a flat tyre and only checked that once we had spoken and i had mentioned that it might be the bead or valve. He also told u at the time that the tyre was fine.
Tyre was only flat due to car being parked up for some time and has never gone flat before.
They are fine to use on them rims mate.