Who owns the mainline? Call them to clean the mainline more often. and send them a copy of the video. If you are getting odors/gases in the house , you have at least one dry trap. Run water down each drain in the house.
Why don't you talk to some of your other engineers? I can't see why anyone would want this? In the US you need to attach to a non-moving part of the structure. You will also have to get the blessing of OSHA. You are attaching to the one most unstable part, the ladder.
The first thing you do is talk to the local inspector. then you do the perc test. It will probably fail, given you comments. Then start looking at mound system.
Could we all send emails to our respective State Lic boards? There should be enough people to hit all 50 states. If you advertise engineering services, you need a Business State Engineering License and a PE with the State Lic. on staff.
If the pipe has been in the ground for 5-10 years, just run a TV camera through it to see if anything has changed. As CVG stated, the live load on the pipe will be very small.
I would use a stainless steel band on the inside of the HDPE and drill in 4 inch anchor bolts using a 2-3 inch stainless washer about every 8 inches around the pipe.
There should be a manhole at the connection to make maintenance easier on the small line. There will be issues with the small pipe coming in below the level of flow in the trunk line.
