Yes. If the analysis is at or above the EPA protocols. Keep in mind there is no safe level of Radon as stated by the EPA. Licensed and qualified mitigation contractors should be contacted to obtain more information and prices.
Yes. Any changes to the foundation can create pathways for Radon to enter the home. At the time of sale it is a good idea to check on building permits and construction that may have had a impact on Radon entry even when a existing mitigation system is present
The correct place to start is in seeing what the Radon level is in the home before going into water testing as the costs increase in testing procedures focusing first on the air that you breath. Water testing is available and should it test at a elevated Radon level it is recommended that you consult about corrective measures and alternatives prior to any corrective action.