[rahimnathwani]

Regarding the second point, failure to deliver is an insurable risk...assess this risk.

Yes, but whoever insures the risk would face the same difficulty in getting enough information to properly assess the risk. I'm not saying it's not doable. However, it might not be doable well enough, and cheaply enough, that there's enough margin of safety for the investor, above the lowest price at which the employee would be willing to 'sell'.

Under what conditions have employers been able to pull back shares that are already vested without being sued into oblivion?

Skype pulled back options that were already vested: http://finance.fortune.cnn.com/2011/06/24/skype-vesting_cont...

I'm not aware of any similar instance for vested shares.