Cylinder misfire, oil loss, multiple repairs, unsure of the problem

Hello, Prius community.

I bought my first Prius (2011, 119k miles) in May 2018. In July I drove cross country for a total of around 9500 miles. One oil change during my trip, no issues whatsoever. When I got back I was due for another oil change and just before I brought it in I got a CEL. Cylinder misfire. Had the ignition coils and spark plugs replaced. One week later I got the low engine oil pressure light. It came on intermittently when braking at low speeds. Oil was low, brought it back in. They replaced the intake manifold gasket and said that was causing the oil loss.

Drove another 5k miles, noticed some oil loss. Then last week my engine knocked on cold start. I topped off the oil and the engine ran quiet. No CEL light, but I pulled codes myself and found P0301, P0303, P0304. I'm not sure if these codes were old and not cleared by the mechanic. If they were new they should've thrown the CEL, right?

Did some poking around on the forum and found the TSB 0010-12 to replace the intake manifold if there's engine knock, oil loss, and misfire codes. Took it to the dealer. They say, "We pulled code P0304. Since the TSB pertains to P0401, the issue is not the EGR. Also, that TSB was made obsolete in April. We can tell you what the problem is if you OK the $150 diagnostic." No thanks. When I pick it up they say the engine needs to be rebuilt due to piston oil rings being worn.

I checked the service history of my car. At 90K miles the PCV valve was replaced, the intake manifold was cleaned, and the intake manifold gasket was replaced. No other repairs related to the intake or engine. I bought this car from the original owner who routinely serviced the car at the dealer.

I'm confused about what's going on here. How does the dealer know the issue is my piston oil rings without doing a diagnostic? My thinking is that if I really do have worn piston oil rings, it would cause frequent misfires and the CEL would stay on. The last time I got a CEL was back in late August.

Thanks for your help.