The seller is not required to install a mitigation system UNLESS the buyer makes that request on an inspection notice. That can be an item that is negotiated, particularly in this market. It is important to use a reputable contractor to install the system so that the radon is actually removed from the home. If the buyer does back out of the contract, the test results must be disclosed to future buyers, so its probably in the seller's best interest to fix the problem the first time.

No, but they do have to disclose the positive radon test in their seller's disclosure and to all future buyers.