JC: I’ve been holding off commenting further but I really have to.
If taking a shower causes a main line sewer to back up, that is a sanitary/plumbing emergency.
Given the time elapsed doesn’t help your case noting the urgency of this problem.
Sounds like the guy is basing the small claims extension date to after he sells. Might be tougher to prove and (find and serve)if he is gone.
You need to come to grips that if he goes you will have to pay for all of the repair yourself.
Get the cost to make the repair and compare it to the legal fee to file against neighbor. This might give you some insight as to what to do now.
Trying to get a new owner to pay will not be easy as he will balk and claim nothing was disclosed and since the problem appears to show up on your property —its your problem alone.
Sounds like you have a bunch a good evidence. Sit down with a real estate atty or two for the free consult.
That will give you clarity on the case and remember if you win the neighbor will pay fees. Might be interesting to find out of you could actually lien his property for the repair costs given the shared sewer. Dunno..