I don't want to kick out my tenant... main line needs repair
Background: 600 sq.ft rental house built on a slab in 1923. The drains appear to run in a straight line from the kitchen, to the bathroom sink, to the toilet. From there it exits the wall and joins the tub drain in a little 'mechanical' closet, runs down to what I'm guessing is a 90 angle, and out of the building. Until it joins the tub, the whole thing is iron, encased in the slab - at least I think so. Does that make sense? Would pics help?
The issue: Two months ago, the kitchen sink backed up, quickly followed by the bathroom sink and toilet. Called in a plumber who couldn't(wouldn't) bring his cutter down the hill, so used a regular snake. Pulled up some dirt and gunk. Everything fine until a few weeks ago. Same issue, accompanied by a faint sewer gas smell. Everything but the tub completely stopped up, and this time required hydrojetting to clear.
Given the toilet involvement - it gurgles when running any faucet - I assumed the problem had to be between the toilet and where it joins the tub, which is accessible from the mechanical room… but a camera revealed a crack and beginnings of an offset somewhere around the bathroom sink. The plumber said he couldn't get past it for fear that it would get stuck, so a month's salary later I still don't know what happens at the business end of the line.
His solution: jackhammer through the slab and replace the entire system, which means displacing a tenant for as long as it takes to redo the floors (polished concrete) and newly laid tile tub surround.
I realize that the whole thing will need replacing. One crack means there are likely others, and that roots/mud can get in through the offset and muck things up further down the line… but even then I'm finding it hard to believe that replacing the section after the toilet wouldn't resolve the issue, at least until my beloved tenant moves on and/or I can afford to spend the $5K.
Am I missing something? Can a partial collapse affect the function of the drains further down the line? Is it possible that a vent is clogged? (the plumbers refused to go up on the roof…)
Thanks in advance for any suggestions, and for reading the whole thing. I can handle replacing the section from the toilet to the cleanout myself, but if it won't help anything... I just don't know what to do.