NO IDLE on SR20VET

Really sorry for the long post, but I think it needed to be clearly explained because of the apparent complexity of the problem(s).

Let's talk about my GTI-R. I owned it for 3 years now and drove it only 1000 km, the first summer.
At first, it was supposed to be a tiny project, it only needed some basic parts and we were good to go. But things went terrible. It's been 3 years and it still not idling.
So, let's start at the beginning. I bought it from a good friend of my roomate. He himself bought it from a guy that claim to have put around 50 000$ on it (including importation fees, taxes, repairs and upgrades). For all the time that the last owner had it, things went pretty good. The engine never had a great idle, but it drove perfectly and pulled very well. He had nothing to do on it until the engine blew out (not sure why!). At that point, he decided to rebuild it and to prepare it to make huge power. So he installed some forged rods and pistons, a fuel pump, a fuel pressure regulator, etc. He was planning to put a huge turbo on it to make in the range of 500-600 whp. The engine was built and installed by a professionnal. The only missing parts were the huge turbo, the manifold and a rad (he sold it, not sure why). Then, the owner decided to sell the car because he lost interest in the project and had another one in mind.

So I bought it and putted in all that was needed. I decided to put the same turbo than the last setup (precision 5130) and to forget about the huge turbo setup. The first start was perfect. We did the last tiny remaining things then I began to daily drive the car. The car drove perfectly, no sign of hesitation, idle was ok even if it was running a little rich, but nothing bad. So I drove it on around 1000 km to break in the engine (it hadn't been drove since it's rebuild). I even went for a weekend in my hometown (250 km away) without any issues. It's even surprising how a car from the early 90's can feels that smooth on the road, almost like a brand new car (not joking).

So, almost a week after that, I took the car to the shop where I work to put some goodies on. It was parked outside for 2 rainy days. After 5 minutes of driving, it became to run VERY rich. It went to 10 on my A/F gauge (the smallest value on the gauge). No matter if I was idling or pulling, it was dangerously rich. So I parked it at the shop.
We tried a lot of things : changed the MAF (two differents), made the injectors cleaned and tested, changed the O2 sensor, changed the ECU, etc. (I propably forgot something).

So, all this work for absolutely nothing. The car was still running extremely rich (on idle only because at this point I didn't wanted to drive it anymore to make things worst). So I called the tuner to know if he could tune it like that and he told me that there's no problem, just bring the car and he'll tune it and it will be fine. That's what I did. And that's what he did. He struggled to make it idle correctly at first, but finally succeed. He made some pulls and everything was alright. The car was still stalling after a pull, but nothing bad. Then one time it stalled and refuse to start again. He tried a couple things, scraped a couple grounds, pulled a couple hoses and suddently it started (i'm not sure why - him neither). He was able to tune it correctly, even if I still had a little A/F mixture problem. Thats why he decided to stop to 347 HP and 364 Ft-Lbs torque, even if the engine and turbo had much more potential than it. He tought it was a bad MAF problem and that changing it will solve the problem.

So I came back home and when I installed my new MAF, I noticed that the piping between the maf and the turbo was loose ! I pulled and it came out...I've forgot to tighten the flange around the pipe...I tightened it and went for a ride. No more rich trouble ! I was so excited that I ran right into a traffic jam without noticing. It really tested the setup because it used to run richer when stopped. I had absolutely no problem in the traffic. I was two corners from home when it began to run like shit. It was idling high, then struggling to idle (had to give more gas so it won't stall). I parked it and it's there since that moment (4 months ago).
Since then we made new grounds directly to the battery, checked for leaks in the turbo/intercooler pipings, checked the TPS and the IACV, changed the spark plugs, checked the fuel pressure regulator, checked the fuel pump, checked the timing, checked for codes on the ECU, etc. (I think I forget something). We checked all the wiring (the better we could) and the only thing we found is a cutted MAF plug wire and a cutted injector wire. What's strange is that even soldering these wires haven't changed anything. By seeing it we could guess that the wiring dried with the time and that we may have to change the entire harness. That's where I am stuck now. I'm currently looking for a good harness, but it will be a lot of trouble to change it, so before to do it i'm asking to you guys, what do you think it is ?

I'm really desperate now...it was supposed to be a tiny project and it's been 3 years and more than 10k $ later and I can't enjoy my car.

(I know that the best upgrade is to install a complete engine control system like AEM does, but I reached my money limit for this project for this year.)

Here's a link to a video showing the issue :

https://youtu.be/i94KQHk0p8Y

Thanks for reading and sorry for the long post.