In matters of health and safety involving a serious lack of information people would be well advised to consult with a local expert that could hopefully help you identify your problem more clearly and assist you with your options.
Personally, if seriously high levels of redon do exist, it may be best to consider other options.
I think you are talking about a duplex home (side by side or up & down). Contact a local Realtor to be your Buyer's Agent. A Buyer's Agent will protect your interests & find homes for you & your mom to look at, all at no cost to you. The Seller pays the Buyer's Agent's commission.... more
Off the market could mean the listing was withdrawn, has expired from the market, is under contract or sold; for an actual status, contact any local agent/realty office as they all have access to the same information.... more