[ossiblue]

Without knowing all the details, my guess would be that the blockage in your neighbor's line is upstream of where you branch into the main line--that's why he has a problem and you don't.

Having another company scope both systems will, probably confirm his is bad, yours is failing, and the main is failing as well. After all, it's a 100 year old system.

Your third question is very important, and you need an answer from the city, not the repair company. It is common for repairs to require code upgrades but this may or may not be one of those times especially if your branch and the main to the sewer are still working.

Your fourth question will likely depend on where, on your property, your line branches over to your neighbor. The difficulty in laying a new line is probably why a 20k estimate was given.

Whatever you do, get a second or third estimate for a new sewer connection.