It's a mystery for my family for over couple months now but discovery couple days ago narrowed down to what the problem could be.

1. There is water standing in the sewer pipe.
2. Septic system has been pumped and water table is low in the tank. It's about 60ft from the house.
3. With moderate usage, sewer pipe fills up all the way to the point where it floods the basement floor through the floor drains.
4. Gurgling in one of the traps indicates the moment when water level passes that branch when filling up, then again gurgling when it comes back.
5. IMPORTANT - 2 days ago noticed big pool of water by the end of drain tile outside. It was about 0F outside and stinky water was still coming out.
6. About 5 days ago, sulphuric acid drain cleaner was dumped into the floor drain, followed by gallon of water (possible connection to #5?)
7. Drain has been snaked 2x now and confirmed that snake ball hits the metal baffle in the septic tank. Very little water goes in the tank.
8. Drain tile runs by the basement floor (about 7ft under ground) and crosses the septic pipe (never seen the pipe, must be below the drain tile)
9. Septic system is circa 1983, assuming it's PVC.
10. There are several big black walnut trees growing around the septic.

I called some plumbers with drain cameras but still waiting to confirm the appointment. They would like to see clear pipe (no crappy water) when running it, but obviously that might be not possible.

What possibly could be the cause?
1. Would S-trapped (collapsed) pipe cause my problem?
2. Separation in pipe (so top edge of incoming part sits just above the bottom edge of following pipe)
3. Tree roots (but then snake passed through and water is still not draining)

Any other ideas or suggestions?
Does it make sense to rent power auger and try to clean it myself before plumber shows up?