OMS - Your point on contamination of Midel with mineral oil is interesting. Depending on the level of contamination this could impact on the potential flash point, and also the future disposal route. I have been advised by contractors to order an additional drum of Midel purely for flushing, which pushes up the costs again.
To be fair, flushing with one spare drum of MIDEL isn't going to totally flush the windings etc - they are (by definition) completely saturated with mineral oil. I once had to ask the question of lindley thompson regarding a mis specification on a transformer - this was a 1MVA unit that had been ordered with mineral but should have had MIDEL on board - in thier considered opinion, it was cheaper and less risky to buy a new transformer (with the appropriate credit for the mineral unit) as the time and effort taken to flush was excessive if you wanted any kind of assurance.
Personally, I would go back to your insurance company and innocently ask the quastion as what stance they would take in the event of a claim where you had (on thier instruction) invalidated all of the declared characteristics of the transformer and an incident/accident occured
Let the wind blow you, across a big floor.