How do I unblock an interceptor chamber?
You are lucky there is a vertical pipe. The teapot lid is often at the bottom of the chamber, so it needs to be pumped out for access. As you suspect, this should enable cleaning out the main house trap. The pipe with the flap that is under water is to access the pipe beyond the trap. Either the pipe beyond or the trap can be the cause of the blockage. Dealing with the pipe beyond likely requires professional service with root cutting equipment.

So lets deal with the trap and hope that fixes it. You need to feed a plumber's snake down the pipe and through the trap to break up the blockage. I've also had luck simply jetting out the trap with a common garden hose with maximum water flow. How successful you are depends on the nature of the blockage. It may also require professional service with the proper equipment.

If you do require assistance, enquire about the cost of removing the main trap. It is typically no longer required by building codes and would eliminate a common trouble spot. Sometimes it's an easy job and other times it means digging up your garden.