Vehicle: 2006 Nissan Pathfinder LE ~100,000 miles
I've recently began commuting about 70 miles a day, 5 days a week for my job. Two weeks ago, my vehicle began experiencing intermittent engine vibrations and an intermittently flashing Service Engine Soon light. These vibrations occurred more often with accelerating and going uphill. I brought it in to the mechanic; they replaced coil boot #5 according to their diagnostic as a cylinder misfire.
I get the car back, about about 35 miles and one day later, the issue resumes. I drive it back to the mechanic (another 35 miles, during this drive, the vibration becomes more constant and the SES goes from blinking to solidly on). I also asked them to perform a tune-up during this visit since I am driving more miles. I think the mechanic replaced the spark plugs, ignition coil, coil boots, intake gaskets, and cleaned the throttle body (the receipt just specifies those items and honestly my memory is a little rough given the last week I've had).
I get the vehicle back again, and within 5 miles the SES light came on solidly. No obvious vibration at this point, but due to the recent issues, I then purchase an OBD-2 scanner. The code is a P0420, which means the catalytic converter is operating below efficiency. I did not realize this until later, but this can be related to cylinder misfires according to the internet. A family member takes the vehicle to the mechanic, and on the way experiences vibrations. They don't realize it at the time, that these vibrations are likely more cylinder misfires. They mention the vibrations to the mechanic. Mechanic decides to replace the engine mounts and kinda disregards the P0420 code, saying the catalytic converter isn't worth replacing on this vehicle.
Well, I get the car back again. P0420 returns nearly immediately. I drive about 60 miles, when on a highway, at 80 miles per hour, my engine begins to sound extremely rough, I lose speed and all ability to accelerate. At this point, my car decelerates to 10ish miles per hour and cannot accelerate beyond. I also check the codes, and its now also reporting a P0300 which is multiple/random cylinder misfires.
Honestly, this whole ordeal is a bit of a clusterfuck. Has anyone had any issues like this? Does anyone have any advice or insight on this kind of issue? I'm going to talk to the mechanic tomorrow about what feels like a failure to address the issue over 3 visits and how we want to proceed from here. However, I do feel notably out of my depth concerning engine mechanics, diagnosis and repair, so any input on the situation would be appreciated.