I am not a teacher. I have a B.A. in Communication and help teach ESL.
This phrase: "...but only if and to the extent that such disruption, despite the exercise of reasonable diligence by SELLER, cannot be or be caused to be prevented, avoided or removed by SELLER" is legal terminology. As such, it is written to cover all possible circumstances.
The best way to decifer it is to break it down into its constituent parts:
There is a potential of some sort of disruption. The mitigation of the disruption, in this case, seems to be up to the Seller. The Seller is to excersise reasonable diligence to stop the disruption.
If the Seller acts with reasonable diligence and,
Then there is some (left out of the quote) way out of the "contract(?)"
- the disruption cannot be prevented by the Seller (by his/her action)
- there is nothing the Seller can do to cause the disruption to be prevented (by the Seller's agent)
- and the disruption cannot be avoided or