All of the above would seem to be credible. If it has any hydrocarbon content whatsoever, I would very strongly recommend a full strip, clean and rebuild of the master cylinder with new seals, as "mineral oil" is extremely damaging to the integrity of the seals, and some may have leaked in from the servo.
You could try testing it for flammability, CAREFULLY!!!!! We don't want singed eyebrows, or worse. Been there, it hurts, and no, it wasn't with any fluid from a Mini, or any other car... Brake fluid burns, as does mineral oil, water obviously does not. I would test by putting some in a metal tray, outdoors, and applying a blowlamp as the source of ignition, because brake fluid and certain oils do not ignite all that easily, except when you don't want them to! (Engine oil and brake fluid are both supposedly more likely to ignite on a hot manifold than leaded petrol, so allegedly is antifreeze.)
However I suspect that what you had in the system was indeed dirty water, drawn in and condensed due to pressure and temperature changes, but proving that may not be easy. You could use a small amount of it in various seperate tests, for example, does it mix with oil, petrol, water or brake fluid? Obviously oil and petrol mix, as does water and brake fluid. Does it mix with alcohol (meths)? Does it dissolve salt, sugar, sodium bicarbonate, and whatever else you have to hand to try?
