P0420 and oil consumption on 200k

I installed a cat shield to prevent my catalytic converter from getting stolen. Now I wish it would have....
After a long road trip, my 2009 Prius, 200k miles, showed a check engine light, with code P0420. The service dept. said I need to fix the catalytic converter - they checked all the other options. The car had been burning oil for some time. I think it is getting worse. Probably 1qt every 1000 miles. I tried 44k cleaning, but it seems it didn't work all that well.
Now the question is what to do.
1. Clean catalytic convertor with spray or taking it off and using various chemicals.
Reports seem to indicate this will not work
2. Get a replacement cat, hopefully for less than my house costs. In theory, if stolen cats are being sold, I could sell mine as stolen, and buy a stolen one for zero cost difference :grin: I will not be able to bring myself to install a cat that doesn't work well
Of course the problem is that if I install a new cat, it will fail soon (probably less than another 200k miles), because of the oil burn.
So I could
3. Ignore it, and hope the car dies before the new catalytic converter does.
4. Try a piston soak as suggested here. I have to say that I'm quite afraid of that. (Though it could help me reach pt 3 above faster.) While doing the piston soak, I could do other cleaning - such as intake manifold and such...
5. Try to get a new (old) cat and a new (old) engine.

I'm still on the first hybrid battery.

 

Thank you! Today is the first night. For now I filled with seafoam. Next night will be different. Maybe your 50/50. How to deal with the fact that half the pistons are closed and half open? I'm not totally sure of the best way to rotate the engine.
I'm also not totally sure how to vacuum it back out. With a syphon? I'll check how deep I can get in there....

 

Thanks!
I'm trying to fix the oil consumption first. If that get's fixed, I'll replace the cat. If not... not sure.
Do you know how well these cats work? Are they just cosmetic, or do they actually do their job, just less well?

 

Thank you! This helps a lot. I poured some Berryman's B12 in, and managed to turn the crank pulley bolt. I think. (see below. Though no 1/2" square drive hole)

 

Things seem to be going well for now. (Well, except for managing to dump B12 over my forehead...) There is the question of exactly how much force is involved when you are supposed to "not force" a big lump of metal. For now it means I can't rotate the crank pully bolt clockwise any more. But it seems my B12 is seeping into the pistons.
Of course the big question is how much damage have I done, and will my Prius ever restart.
But if it all goes well, maybe I should write a "Piston soaking for dummies (or people who never changed a spark plug)" guide.

 

Not so good. Maybe false alarm?
After finishing the soak, mainly with Berryman's B12, I changed the oil, cleaned the MAF sensor and the throttle plate, changed spark plugs. The car did start, lots of white smoke. I drove around a bit, and then got a red triangle, together with
P3190
P0172
P2196
I rolled the car back down the hill, it made it home. Now I'm not sure what to do. Are these codes expected? Should I just bring it in to be checked? Should I have it towed to be checked?

I disconnected the 12V battery, waited a few minutes, and reconnected it.
When I turned the car back on, all seemed good. No more white smoke, no red triangle, no fault codes.
Let's see how long this lasts...

 

OK, update. About 4 refuels later, the oil level is still up high. Normally we had to refill oil every one or two visits to the gas station. I think it was a success!

 

Final update. It seems we currently don't need top-offs between oil changes. Full sucess! We're still checking the oil every time we refuel. Hard to break the habit/fear.