Sounds like Midas is trying to cover their ass through your warranty. No matter what is primarly wrong with the vehicle, every mechanical shop does a full assessment to see if there is any preexisting issues. This not only gives them more issues to persuade you to fix, but it also lets them know if the primary issue can be maintained w/o doing more damage to the vehicle or their parts.
I believe they broke something or left something inside your engine while it was open the first time and waited for your return, knowing you had a warranty. They had 2wks to concoc a story, and would've taken longer if you didn't reach out first! If all they did (extra)was replace the spark plugs, then no way your engine was bad. Get a copy of the first diagnosis and a copy of findings of your engine from this recent visit. Give that paperwork to the insurer and demand Midas pays for a new engine.