So, the mini is killing me.

I'm getting the following codes:

P0507 Idle Air Control System Higher Than Expected
P129D Manifold Absolute Pressure Maximum Pressure Implausible
P1497 Downstream throttle air leak
P0300 Cylinder Misfire
P0304 Cylinder Misfire
P0301 Cylinder Misfire
P0302 Cylinder Misfire

The following has been done:
Today:
New Valve Cover
New MAP Sensor

September - note that the car ran fine for several weeks after this was completed.
Head Gasket
Rx Oil Catch Can installed

I've gone through the hoses from the air box all the way to the intake and haven't found any of them to be loose. I'm ready to drive this ****ing thing into the ocean.

 

I gotta admit to being kinda slow sometimes, but I can't determine from your description what the sequence of events was. I'll guess that head gasket and OCC was first, then you got the codes, and last a new valve cover and MAP sensor (there's 2 MAP sensors, which one was replaced?), and you're still getting the codes? Also, note that it's been a few days since you posted this --- someone usually offers a solution or asks a question if they understand the problem. Any other mods? Most of the common parts failures fixed already? Are you the original owner, or did you buy someone else's problems?

Hopefully, the head gasket was done by someone familiar with the N14 engine --- cam timing is critical! If the OCC is connected improperly, the resultant codes could be described as your 1st three listed. Did your "hose check" include the passenger-side PCV / OCC hose connection --- especially at the manifold? Note that there's a few different ways to connect an OCC --- some better than others, and with varying results.

You might also test the wastegate operation --- if no wastegate, you'll get lotsa misfires, excess MAP, and maybe blown out (not loose) hoses.

 

Problem solved. This morning was the first commute after putting in the solution. As I don't check my email that often, seeing your post to respond to it this morning is damned good timing.

The order things happened is:
Install ebay Forgery FMIC (April) car runs fine for months.
Install OCC result is car runs fine.
Valve cleaning result is cylinder with walnut shell particles in it from forgetting to roll the engine over on #4 cylinder.
Pull head, remove walnut shell, clean surfaces of carbon build-up.
Reassemble engine (new head gasket, head bolts, cam bolts, etc.) result is car runs fine. Actually better than before due to valve cleaning.
6 weeks pass with the car running fine.
Car starts throwing codes listed.
Notice oil on top of the valve cover and in two center bolt holes.
Look up codes and note that possible causes seem to point to valve cover and MAP sensor.
MAP is cheap enough that I just purchase it anyway rather than just dot he valve cover and have to wait for another part to be delivered.
Replace valve cover and MAP Sensor.
Car keeps throwing codes.
Visual inspection of every hose and connection throughout the intake tract reveals no loose connections.
Verify timing is good.
Get fed up with throwing money at this damn car.
Investigate trading it in even though I'm upside down on the loan.
Post thread asking if anyone else has seen this or has a direction to point me to.
Talk to co-worker gear-head for other possible solutions I've overlooked.
Create a home-made smoke machine.
Discover that the problem location was behind a hose making it not visible from below the engine.

Problem was an intake manifold gasket.

Apparently, during reassembly the gasket came out of the groove on the intake manifold. My guess is that it was pinched just enough on the very edge to maintain intake manifold pressure for those six weeks it ran fine. Then, either through heat-cool cycle or just the pressure generated through a high-speed blast it finally broke through and started throwing the codes. Car would still run fine but idle super rough. Power didn't seem affected, driveability was fine except at stop signs/lights.

As for the other specific questions...
I definitely inherited someone else's problems; it's called "a Mini Cooper".
I did the head job (Bentley Manual is a must!) and the car ran fine for weeks afterward.
OCC hoses were part of the overall check for anything that came loose as was the drainage valve being knocked open.
I did not check the wastegate. Had I seen a suggestion for that prior to solving the issue I would have done so. Good to note that for anyone searching for this or similar problems in the future.
The gear-head I work with also suggested checking the IAC unit but I was having trouble finding info about it for the N14 so I put that aside thinking I would revisit it after the smoke test.

 

" Problem was an intake manifold gasket. " Yea the install and the distance to the seat is minimal so it can happen, glad you narrowed it down.

" The gear-head I work with also suggested checking the IAC unit but I was having trouble finding info about it for the N14 so I put that aside thinking I would revisit it after the smoke test. "

Some info here, its controlled edk : https://www.northamericanmotoring.co...issue-iac.html

So possible throttle body seal or stuck throttle body gummed up with oil / moisture from the PVC.