I'm happy to report another positive data point toward the project! I'll be returning to my local inspection shop tomorrow morning to get my inspection paperwork. I essentially followed the protocol outlined in this thread.
A couple points to hopefully help clarify or append to the discussion:
I bought an 8oz package of oxalic acid from Amazon for $9. It was just barely enough for my 30 qt turkey fryer pot purchased from Craigslist. (I mixed with 7 gallons of water... 8oz oxalic acid crystals was right at 350mL). I bought two pounds of citric acid as it was only a few pennies more for a larger size and apparently it has all sorts of household uses which may have various benefits over premixed products. I was going to buy a fuel pump and circulate the fluid through converter, but didn't. If I have to do it again I think I will. I just threw the pot on the grill which worked very well and made it easy to control the temperature (it takes a long time for 7 gallons of water to drastically change temp).
So, had everything gone right, I'd have been in about $40.
....unfortunately...
one of the shoulder bolts fixing the cat inlet to the exhaust manifold collector outlet stripped on the way
out. I've never seen this one before. I had the impact straight on it and had to fight it the whole way toggling between forward and reverse after heating the weldnut a couple times with the map gas torch. It appears material from the bolt threads was left deposited in the nut. Hindsight being what it is, I guess I should have done the 'ol wax on the hot threads trick. After getting the $30 fastener from local Honda dealer and tapping the weldnut threads, we got back on track.
Of course I sort of left out the part about how when I tried to find the thread pitch against the other bolt I incorrectly thought it to be the larger M9x1.25 which I had a helluva time threading into the M8x1.25 welnut on the car all upside-down like reaching through the subframe... so wanting to be sure not to screw it up worse, I elected to pull the exhaust manifold (plus I'd noticed a little valvetrain noise lately and wanted to check valve clearances anyway).
Attached is an updated downstream O2 sensor graph. I sure wish they unlocked the upstream sensor for general use. It clearly is much more steady, but maybe not as much as I had expected. That being said, I've only had two 25 mile runs and it clearly continuously improved throughout my first run this morning. I got the happy green "Complete" for Catalyst after only about 10 miles. Side note, it appears one may not be able to pass the Evaporative system diagnostic if not first passing the Catalyst test. That one cleared on my way home this afternoon (the next drive cycle).
Also, I did find an exhaust temp sensor on my car "Cat B1S1" and added that to my display. I guess this is measured by the upstream O2 sensor; without a sensor just downstream of the CAT it doesn't seem too helpful to aid in detecting a poorly performing cat. I found some interesting notes here:
https://www.aa1car.com/library/converter.htm " In many late-model engines with multipoint fuel injection, combustion is so clean that the converter has little to do and the difference between the inlet and outlet temperature may only be 30 degrees F at 2,500 rpm - which is a lot less than the old rule of thumb that says a good converter should show at least a 100 degree F difference fore and aft at cruise."
Value of answering the neighbor kids (while wearing goggles, gloves, and high-end mask and dipping the assembly into the pot on the grill), "it's a vat of acid" [which I'm using to save hundreds of dollars while hopefully learning a thing or two] and then cooking dinner next to it... Priceless (unless I get cancer, of course.).