Service Engine Light On problems of the 2013 Mitsubishi Outlander

The catalytic converter was recalled and replaced in November of 2017, but now the exhaust manifold is rusted from the inside which caused bits to break off and fall into the new catalytic converter. Now the exhaust manifold needs to be replaced (there are only 4 parts in the nation for this??) and the new, less than a year old, catalytic converter has to be cleaned out of all the rusty metal bits. There is a rattling noise in the engine when idle (rpms jump up and down slightly in idle), quiets a bit once you hit 2-3k rpms and then is loud again around 1-2k rpms. The exhaust manifold is nearly a thousand dollars to replace! I don't live near the ocean or a place where it snows frequently. My 5 year old car shouldn't have rust in the exhaust manifold especially considering the other recall just last year. Service engine soon light has been on for weeks since the problem started. They told me it's out of my warranty because my car is at 75k miles.