04 Hyundai XG350 - Completely messed up after mechanic worked on it. What do I do?

I purchased a Hyundai XG350 from a private seller a day before bringing it into a local chain mechanic. The car was nearly flawless - 90,000 miles, well maintained with all service records. Aside from an oil change and a leaky CV boot, there was nothing wrong. I had the car checked out by another mechanic friend who told me that everything looked great aside from the CV boot, so I can verify this.

We brought it in for the oil change, and the shop gave us their inspection that they give with all service. Only thing noted was the CV boot. We knew the timing belt would need replacement, so we inquired how much it would cost to replace along with its associated parts. They gave us a fair estimate for the work along with fixing the leaky CV boot.

I asked the guy who oversees the shop if everything else looked good to which he responded "yep!". Okay. Great. Clean bill of health like we thought.

We left the car there that day and he assured us the work could be completed by the end of the next day.

The next day came and went. When inquiring about this at the end of the day, he responded that they needed some more time to complete the work and that it would be done the next day, and they would call us when it was done. We were annoyed, but accepted this.

The next day came. We called at 4pm to be told "yeah it's ready to go"...gee, thanks for calling us.

We pay for the repairs and take it home. The car runs great. It runs great the next day going to work (about a 36 mile drive). It runs great again the following Monday...almost.

We go to leave to the grocery store at 6pm and the car starts and violently shakes. The RPM gauge goes wild and the check engine light comes on. That's definitely not normal. Run a code check and it's throwing a P0300, P0301, P0303, and P0305. Fearing the worst, we have it towed in the next morning. The guy at the counter tells me "we'll look at it!" and rushes on to the next customer.

We call for an update around 5 that day after not hearing anything. Guy says they just got it into the shop to look at and they need to check the timing amongst other things. They say they'll call us the next day.

Tuesday rolls around and by 5 they haven't called. We call, he tells us they don't know what's wrong and they need another day.

Wednesday we call and are greeted by a technician who doesn't know about the car. He calls the guy we've been talking to who then tells us they need another day.

Thursday we get a call from the guy. He tells us there is now a knock coming from the rear of the engine "that sounds like metal on metal". They don't know what's causing it and need another day. Do note that there was no knocking when we brought it in on Tuesday.

Friday we call him, and he tells us they're still trying to determine the source of the knock....

And throws in a bombshell...where at this point I lost my shit.

He tells us over the phone that "you know those check engine codes we cleared when you brought it in the first time? We're trying to figure out what is causing that"

Excuse me?

We ask him what he means by the first time:

"You know, when you brought it in for the timing belt replacement?"

WHAT?!

Excuse me sir, there was no check engine code when we brought it in the first time. I brought it in to be fixed with a check engine code AFTER you worked on it. The car was flawless beforehand.

You told us yourself that everything looked good when we brought it in. There is no mention on the repair receipt of any check engine light or misfire that was diagnosed or any indication that there was anything wrong. Because there was nothing wrong.

I wouldn't have spent thousands of dollars on a car the day before we brought it in if there was a check engine code. I checked with a code scanner twice before buying the car.

Not to mention the misfire was jarring and completely noticeable once it occurred. There was absolutely no way I just didn't happen to notice it before. And no way it wouldn't have been mentioned when they did their inspection.

We call him out on this and his response is

"Right. Okay. Well, just give me another day to look at it."

I cannot believe that someone I trusted with over a thousand dollars would straight up lie to me to avoid responsibility for this issue. I was absolutely flabbergasted and disgusted by his response.

But then I read reviews elsewhere. It looks like somebody else had a similar experience with a timing belt replacement and his claim that it wasn't their fault; the owner had performed work on the vehicle after they replaced the belt and that caused it to break.

I can say for certain in my case that there was no work done on this vehicle at all after he and his team handled it.

And based on the multitude of service records I have for the vehicle (and the fact that the car was nearly perfect beforehand), I know this issue didn't occur because of a lack of maintenance.

Boy, that seems to narrow it down a lot...oh. Let's take a look under the hood. Gee, the timing belt cover is all cracked now.

Gee, there's a bolt missing where the engine cover is held on.

Gee, these wires are now crimped between the two halves of the timing belt cover.

I wonder where the problem came from?

So now I'm out over $4300 after buying this car and having this place do the work.

I have no idea what to do right now. I'm so angry, and because I'm not a mechanic I feel like a court wouldn't take me seriously if I say there was no problem when we brought it in and this mechanic is making up lies to avoid responsibility.

Any advice would be much appreciated.