1. Yes, 'original inventory required' means that the fare class you are booked in must be available on the flight you wish to change to.

2. This effectively means you cant keep changing the flight. You get one fee-free change, after which you will have to pay the fee if you wish to change again even if the waiver is still in effect.

Ir original inventory is required and unavailable, then you would pay the difference in fare but would be exempted from the 150/200/250 change fee.