1. Most swirl flap blanks (at least the ones sold around here) are a bit undersized, relying on the O-ring having to take up more slack than it should, which leads to the O-rings failing after a while, resulting in a boost leak. Some people add sealant on there, how good of an idea that is is up for debate. Unscrewing the flap itself from the shaft of the flap is another option (slightly different than what was done on the car in the video at the start, where the shafts were cut completely), that leaves an OEM quality seal. However warrants you leave the actuator shaft in place, so it prevents the flap shafts from coming out. Personally, I opted for the more expensive blanks plus sealant after the first set of blanks failed as I mentioned. Either way, you should keep an eye on whether the blanks have not become leaky after a while. That would be indicated with oil spray around the area.
2. Must make sure all of the seals with metal washers are in place. The ones that go between the manifold and the M7 studs especially. You can see one is missing in the video, maybe he put it back on, or perhaps it was lost. It wasn't mentioned. Either way, they must all be there, otherwise as you tighten it up, it will crook over and likely not seal properly.
3. You can leave the vacuum solenoid for the flaps in place, just loop its inlet/outlet together with a bit of hose to prevent dirt from getting in. That way you don't have a dangling connector and don't have to code it out to get rid of errors. Blank off the vacuum line that went to the flap valve close to the vacuum splitter. A bolt with a smooth part of the shaft will work. A bolt with full thread can leave a spiral path along the thread and cause a leak.
4. While you have the manifold off, it would be a good idea to replace the 3mm (OEM uses 3.5x2 hose; 3mm and 3.2mm work fine) vacuum lines that go to the vacuum splitter under it. Especially if the vacuum hoses look like in the video, and even more if you have a car with a vacuum-actuated turbo. 5-6m of the hose is enough to do all of them and it doesn't take long when you already have everything off.
5. I would not use PTFE tape on there, it might help a little bit, but overall this is not really its purpose and it might as well just tear apart. Following up on my previous comment, the 'more expensive' blanks seem to have failed since as well. Maybe gooping them up in RTV to silly levels would improve the situation, but at this point, I think aftermarket blanks are just a waste of time, as I've tried two different sets and both have failed in a fairly short term. Replacing the inlet manifold with one that had no flaps and therefore no holes to plug is the best way. That can be done on E46, not sure about 120d. Other than that I'd try using the OEM flaps just with the removed flap from the shaft. The OEM swirl flap bodies seem to be of much better quality.
6. Vairas