Single family home, built 1994 (I understand most likely all PVC piping due to age), City sewer
Background, May or may not be relevant:
About 7 weeks ago There was a water main break and while repairing the city "accidentally tapped into the sewer line" collapsing the sewer (collapsed sewer was repaired). This water main/sewer area is located about 75 feet south of our property (sanitary sewer runs north past our house). Our lateral is the only one that empties into about an 8 inch sanitary sewer line between the area where the collapse occurred and the next manhole which is located 10 feet north of our lateral outlet.
Our problem started about 5 weeks ago. Took a shower on main level then was running the wash machine on basement level, heard a violent boiling/bubbling noise coming from the basement toilet. Found the laundry room flooded from the basement floor drain. The water did go back down the drain within minutes.
Rotor Rooter attempted to open lateral line. They were able to feed their snakes/augers through over 100 feet without any resistance but the drain was still backing up. 6 hours and a very large bill later, after several people attempting to manually open the drain, they jetted it out. They claimed to be "200 feet out" by the time they were able to clear the line. The distance of our lateral to the sewer should be about 70 feet. At any rate, the drain stayed clear until a few days ago. Used water normally, showers, dishwasher, laundry without any issues.
A few days ago, Again after a long shower on main level, then doing laundry, the infamous toilet "boiling" then the flooding from the basement floor drain. Called the city and they again claimed that it wasn't an issue with their sewer and it must be a problem with our lateral. However, they did come out and jet out the sanitary sewer lines both north and south of our lateral outlet just in case (I had asked if it was possible that debris had entered sewer during collapse). Had been cautiously using water since then for past few days, a shower a day, dishwasher, a load of laundry all spaced out. Then tonight, after draining a full tub of water from main level tub no issue but as soon as starting the laundry right afterwards, here we go again.
Could this be an issue with the venting that goes out to the roof? All the snow/ice has melted now so if it is a venting issue it wouldn't be due to ice or snow. If it was a venting issue, if both clean out caps are removed should that create an alternative vent in the system and allow water to drain freely? Rotor rooter suggests putting a camera into the lateral. If they were able to easily go through the lateral, how likely would it be that the integrity of a PVC pipe would be causing the issue?
I really appreciate any insight anybody may have to offer. I have learned more about plumbing in the past couple of months than I had ever imagined I would have but I really need some expert insight on this issue.