Definitely get a second opinion. Everything about this sounds fishy.

You could have s stuck ring or two, but there are solvents that you can buy that have a better-than-even chance of freeing up stuck rings.

The rings are not worn out unless someone went extended periods without changing the oil.

A different engine won't resolve the problem of all those control system components at are apparently failing, unless you get an engine from the junk yard and bring the electronics also.

Keep in mind that the diagnostic codes stored in the computer are not perfect. Often, a failure in one component will cause multiple error codes to be stored, and there is no guarantee that the actual failure will be one of the codes.

Were those percentages leak-down numbers? Did the mechanic confirm that it was leaking to the crankcase rather then through the valves? Does the car consume oil or smoke blue smoke? Ring failures are generally not instantaneous events.

I'd be looking for problems in the wiring and connections, and talking to another mechanic.