You will have to take a breathe here. Something has to be done. Since you are dealing with the first company that gave you their very high estimate I would have them explain exactly what they are proposing for that $15,000?
Tell your neighbor, the plumber, that you are very concerned and Ask for a detailed plumbing proposal plan so you can look at it and understand exactly what that $15,000 will be doing. Yu will be able to use this proposal to compare it to other proposals and other quotes.
I would also, after I got their proposal, and read their proposal, then tell them I would like to do that but I can not afford to do that. Then ask them if there are any alternative less expensive approaches to solving this problem and see what they are. See what the new lower proposals will address and how much that will cost. In the interum I would start speaking to other Plumbing companies and see what they propose and what they will charge. There are also some master plumbers on BP that I would ask for advise on how to proceed. You can send them copies of the proposal and any pictures.
I would also contact the original Inspection company and get their reccomendations
You are at the start of a research plumbing problem. You need expert advise and you need someone that is qualified to do the Plumbing repair. Good luck and let us know plumber by plumber because this is not an unusual problem and we alll need to learn.