She is running, good!!! And it was all my mistake. Please indulge yourself.
Started today with reading codes again, none had come back. That felt good!
Tested with MAF disconnected, still ran like crap. Tested with my other EZL, no difference.
With those three things crossed off, I moved on to the injectors. Placed a screwdriver on each injector and listened for the clicking sounds, they were there on all of them, so my injectors are working.
On to sparks. I disconnected the plug cables from the spark plugs and put in a spark plug in the cable, just to observe the spark. I did this on each of the plugs, and quickly noticed an either weak or none existing spark on several of them, in the end four, all belonging to the right distributor. Now I'm onto something!
Here comes the funny part. I disconnected the coil wire from coil and distributor, and replaced it with another coil cable from my dads 500E. Started the car, and it still ran poorly, but I could hear a strong spark sound from the engine bay. So went to have a look, and lo and behold, my "disconnected" cable was producing a big long spark. There's only one conclusion I could draw from this, and that is that it's still connected to the coil of course, meaning that I have indeed connected the cables wrong when I reassembled after the intake manifold job.
Turns out that I had connected the coil cable to the nr 1 cylinder at the distributor, and the nr 1 cylinder to the coil at the distributor...
When I reassembled the cables upon completing the intake manifold job, I had them all marked, I had a schematic, I laid them all out on the engine before tucking them in, double and even triple-checking that I had it right, which I obviously didn't have. I ended up mixing two plugs at the distributor after all precautions taken.
Connected them correct, and vroooooom she started and idled immaculate.
One silly mistake made all this trouble.
But today I finally got to drive the car on the road, took a short trip of around 15 kilometers, even made it out on the highway. The car runs really strong!
So now, all I have to do is change the engine oil, filter, attach the skid plate under the engine, and then see what happens at the MOT/emissions test. I'll get that done Monday!
I'm really happy that it was just a simple mistake on my end, it didn't cost any money.
I have to say thank you to all of you guys contributing to finding the fault. Sorry that it was a major cock up on my part...