I believe that the sewer gas is coming from the 2 sewer vent stacks on the roof. I can smell the sewer gas everywhere outside. The front yard, backyard, side of house is blanketed and sitting in car on the street in front of house can smell. It so gross! I put a charcoal filter on one vent and it did not help. Then put 4 foot pcv pipe extention on the other one today. I can't tell yet if that did anything yet as house is so full of gas it might be coming out the open windows. Ordered another charcoal filter coming later in the week.
I think that the wind currents inversion drop the gas staight down to the ground and through open basement window and other open windows and wafting all over the place, instead of going up into the atmosphere. Gas being heavier than the air.
The city has checked evrything out and said sewer positive flow and ok. Nothing they can do.
Ill ask the plumber about the 2 things you mentioned. He doesn't thing that the problem is the pipes in the front yard. And that lots of old cast iron pipes settle a bit. Everyone in town would have sewr gas all over if thats the case. I didn't replace the other drain pipes in the basement draining from the kitchen. He didn't camera them but since it kitchen water didnt think they were the problem