I am sorry I missed this thread the first time you posted.

Based on EVERYTHING you have done, I have a hard time believing the issue is anything that hasn't been addressed. And you have spent a lot of money from the sounds of it. (I would verify your duct tape job) and maybe even replace the pipe with a cold air intake pipe.. (it will work, ill get back to this).

I worry about the procedure you used for the cat. I have never heard of soaking the cat in water to clean it. As well as the suggestion of using higher grade fuel, or fuel additive. I don't believe either of these are necessary, nor will the benefit you. They can also worsen the problem.

Two things I have found with my car, NGK copper plugs are all I would run. They were used OEM, and eliminated a lot of issues. The only problem with them, they absolutely must be replaced @ 20k miles.

I had replaced both of my o2 sensors. Mine were under warranty when they went so I didn't use aftermarket. IIRC oem was the NGK? I cannot remember though. Anyhow, in my experience with most cars, if you do not use OEM you need to "match" brands or specs.. I always tell people to not buy the splice in o2 sensors to avoid getting messed up.

There is only one thing that kills CATs on a properly sealed engine (no oil of coolant getting in cylinder). Unburned fuel. This is where the repair job to your intake pipe comes in. Unmetered air entering the engine will cause the o2 to discover a lean condition and dump more fuel. If you are running high octane, and fuel additives this just adds to the problem. If you think of the engine ECU as operating a venn diagram where optimally all the sensors point to the same area = no codes, good running engine. Each sensor can be a little off and still in range. But they do effect other sensors. When you replace your sensors as they go bad, the union area of the venn diagram changes a little. Each sensor individually can be in their "good reading" zone, but the overlap (union) of the sensors could be off. This effects the fuel map of the ECU.

Anyhow, in my opinion. I do not know if your cat was bad. But you "cleaned" a clogged cat, a cat can also no longer efficiently oxidize particles. This is generally a hollowed out cat, where the ceramic core is missing. I do not know what you tried to fix.
Anyhow, I will try to explain what could be going on in my opinion, using some numbers.. think of the venn diagram.

The three sensors re primary o2, secondly o2, and the MAF. The common signal from these is around 0-5. Individually 0-5 is a good sensor. However, aftermarket parts tend to have a range "close enough" like 0.1-4.9, or -0.1-5.1. When everything is running perfect, all the sensors will be down the middle, or 3 by coding not by reading. However, if two sensors are running around the 5 mark, and the third is down by 1. That would be the sensor throwing the code. But it could be the issue of one of the other sensors staying in the 5 corner (if you look at a Venn diagram, 2 sensors can still be in union while the 3rd is not loner in the union, it can only adjust as far as its circle allows). There can be nothing wrong with that sensor, but that is the code you will get because it doesn't keep in range with the other two sensors. This is why I say fix the MAF/intake issue.

Onto the next possibility, the secondary o2 should "always be half" of the primary o2 to show cat efficiency, again in code not the voltage. But in live data, the voltage change should be parallel. Again, they can both be reading between that 0-5. But if the secondary o2 is only work 3-5 and the primary is sitting at 1, you will get a code. The primary o2 will never throw a code on a good running engine. The secondary will throw a code when it doesn't stay parallel to the primary. However, if the primary o2 is reading in a range to compensate for a lean condition, and the secondary cannot adjust, it will be the secondary that throws the code.

After a rather lengthy explanation, I would say put NGK coppers back in, and fix your maf/intake. (Buy a short ram intake on eBay for cheap) you could even get a flexible one from auto zone and keep the airbag if that is an issue). Run the 87 octane the car is deigned for, and do not use any fuel additives. After you do that, see what happens and post back.