Good work Jim!
I must be missing something regarding your inline filter problem as to the reason why you came up with this setup.
I use 2 inline pool filters (dual wand setup), each with the stainless steel cylinder, with a knee-high inside, and a hanging nylon sock inside the recovery tank, and knee-high over the APO filter which is also inside on the bottom of the recovery tank. I drilled five 1/2" holes into the black hard bottom of the stainless cylinder so the airflow would not be diverted around the bottom of the cylinder.
Before we start a job, I check the filters from the previous job, If it is OK, I proceed. If not, I change the knee high. It takes less than a minute and do not have to clean anything out.
Occasionally I will rinse out the inline filter - maybe once a week, not that rinsing it out improves anything other than it's appearance.
During the busy season, we can go for weeks without any need to do anything at all inside the recovery tank. We run 5 days a week, 2-5 jobs a day with this setup.
With the APO, we don't even have to stop cleaning or dump the recovery tank, we just need to change the inline knee-high when the vacuum drops a little.
I can rinse off the APO knee-high (without having to lift it from the bottom) with a garden hose because all of the crud will be on the outside of the knee-high, not inside. I rinse it out about every 3 months. I have never heard of any other APO that will function flawlessly through 100 jobs with no attention to it's filter!
I use this same setup with my ETM.
I have used this setup now for 15 years without fail.