As has been said, the pilot should not have to prove anything. The Flight Safety department will have enough data available, both recorded technical data and airfield environmental data, to make a reasonable assessment of what occurred. They should also be able to asses the accuracy of the IRS derived ground speed. The pilot should also be given access to this data and make his/her own assessment.
The FS department's only interest should be what, why and do we need to adjust our procedures or training or both.