Starting with two untoned coins of the same type, will a worn coin begin to tone as rapidly as a mint state coin (all other factors being equal)?
In theory, yes; the rate of silver tarnish is governed largely by the amount of sulfur in the air; if they're in the same environment, they should tone the same.
In practice, there would be discrepancies - a "worn" coin should already be partially toned; A "mint state" coin might have oil or other substances on the surface that could alter the toning pattern. It would only be a "fair test" if both the worn and unc coins were washed and dipped prior to beginning the test.
It's also been my observation (admittedly with my mother's silver spoon collection, not my coins) that a silver object that's been "dipped" will retone faster and uglier than a "pure" one that has never been dipped or cleaned. I suspect the dipping leaves "holes" in the surface that attract sulfur more readily.
If one of those beautifully toned coins (that others have posted shots of) were to be cracked out of it's slab and rubbed on as if it were in circulation, what would be the result? Cannot toning be removed by use (wear)?
Yes, toning will "rub off" - it's a very thin layer, and is easily disturbed; silver coins certainly didn't come in technicolour when they were in everyday use. Even thick grey-black tarnish can come off in circulation; ask anyone who was around when silver coins were in everyday use - circulated coins would almost always be dull grey in colour, from constantly having their tarnish rubbed away. Bank tellers and shopkeepers who handled lots of silver coinage would come away with blackened hands at the end of the day; the black stuff would be silver sulfide, the stuff tarnish is made of.
If so, would the toning return to it's present state eventually, or would that depend on the about of rubbing that is applied now?
Toning colour is purely a side-effect of the thickness of the silver sulfide layer; it's a thin film effect
, just like a soap bubble or oil floating on water. The thinnest layers are yellow; then red, green, blue, and finally black. You can in theory get any colour you please on a coin, simply by exposing it to the wrong environment for just the right amount of time.
It should also be pointed out that toning colour is not long-term stable; if not kept in an absolutely airtight container or holder, the colour progression I outlined above will happen. Your silver coins that are beautiful and blue today are going to be black in a decade's time, if you leave them exposed to open air. Even a slab will slow down the progression greatly, but will not stop it completely. I suspect that the Morgan posted by fyimo in the old PCGS slab may not have looked like that when it was originally slabbed; those old slab types aren't nearly as airtight as the modern versions.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis