Modern plumbing codes generally forbid double trapping. Since each individual fixture has it's own trap, the DWV piping has none. Toilets have built-in traps.

I looked into the trenchless options, they don't really work for our scenario. First, the existing line might not be in good enough condition to support trenchless replacement. Also, while we can dig a whole at the city side, the lines terminate under the slab so there's no way easy way to create an entry point for the new line. There's also several joints under the slab, and we'd have to tear out the driveway to get to the existing line on one side. So far, our estimates are $12k (to tunnel to the two known breaks and replace only those portions) or $20k (to tunnel the entire width of the house and replace the entire system). A SEPARATE OPTION: I'm admittedly no expert, but I see a way to run two new lines - one up each exterior wall) which would serve the entire house with minor tunneling. The disadvantage is that the sewer line wouldn't be under the house, it would be buried under the strip of grass on either side. One company says they wouldn't do it that way because the line isn't protected by the house and could break more easily. To that, I say that if it breaks, it would be a MUCH easier fix than if a line were to break under the slab again. Is there any reason a new line can't be run outside the exterior walls of the house?

Would a crushed pipe cause backup? More likely to have flushing problems--waste just wouldn't exit. If you have heavy rains, the stormwater may overwhelm the city sewers. They may then backflow into the house sewer (although they shouldn't). I would talk to the city. Perhaps you can get a backflow valve installed by them. Your neighbor's pipes should run separately into the sewer, although you never know. But same issue there--you shouldn't get backflow from them if they have a broken pipe, unless the broken pipe is downstream from where yours connects with theirs (assuming that's the design).

yes, I learned the hard way on this one and I wish I had given more thought and research to the question the first time through. I let a plumber talk me into doing the relining and was persuaded that this would save the yard from getting torn up. Now 5 years later I find that I can get a new sewer line put in for much less than I thought. I got a bid of about $2k for 42 feet of sewer, house to street, 7 to 8 feet down. That's a lot of digging. I found a good company, though Angie's list. They have done the work and cleaned up very thoroughly. I am going to have my own mason come in and do the concrete work on the walkway; he knows how to match what he did earlier. I'm poorer but wiser. I think the lesson here is to avoid doing the lining. I just don't see any good reason to do it that way.