It seems like the first thing you need to determine is whether or not coolant is in the cylinders. Besides reading the ECM codes, there are various tests shops can do that can determine whether there is any path between the coolant and the cylinders. One is called a “leak-down” test. Another tests the chemistry of the coolant itself for signs of exhaust gasses. And they might suggest a compression test. I wouldn’t expect any of these tests to be super-expensive. Maybe ask if one or more of these tests be done to confirm/disprove the coolant-in-cylinder theory.
Once you know that, one way or the other, then you’ll have the knowledge you need to procede.
Best of luck.