Somehow one of the nine blades in the aperture of my Nikon 14-24mm f/2.8G has come fully loose and can be seen laying turned sideways when I look through the lens. I don’t want to pay to repair it, and I am quite technically competent. Before I really disassemble the lens further, I have questions. Does anyone know how difficult the repair is? Can I repair this blade that’s come loose on my own, or do I have to replace the whole aperture-diaphragm-assembly?
The biggest problem with repairing Nikon lenses yourself is that Nikon no longer sells replacement parts to anyone outsides Nikon's authorized service network, including third party repair shops. To make matters worse, as of March 31, 2020, Nikon is not renewing agreements with any current Nikon authorized outside repair shops that are not owned by Nikon. There are presently twelve such shops throughout the United States that will no longer have access to Nikon parts, service manuals, calibration software, etc. This is compared to only two Nikon owned service centers in the U.S. Nikon is also instituting the same policy in other world areas.
That being the case, your only possible source for another aperture assembly is to scavenge a used part out of another copy of the same lens, find a third party repair shop that still has old parts stock from before Nikon stopped selling parts back around 2012, or contact one of the existing authorized outside repair shops that can still get parts until March 31, 2020. I'm guessing they won't sell you the part without installing it, either.
But even if you could get a replacement aperture assembly, if you do not know what you're doing inside a modern lens, I would not recommend making a Nikkor AF-S 14-24mm f/2.4G your first. It's a fairly complex lens, and also a fairly expensive one. There are a lot of things that you can unknowingly do to make things worse. Ribbon cables are all too easy to tear. Some disassembly steps require sophisticated optical alignment equipment to put things back together properly. Focus position sensors can be damaged just by being touched with bare fingers. And so on.
Once you take it apart yourself, most repair shops will not even consider putting it back together for you if you can't reassemble it yourself.
My advice is to find a repair shop close to you that can fix it, and send it to them quickly, before it's too late at the end of March, 2020.
I find 'mikeno62' has a lot of useful videos on lens repair on Youtube.
In the following video he does a repair on loose aperture blades of the AF-s VR Micro-Nikkor 105mm 1:2.8 G ED:
It seems to be an extremely complicated repair, and you probably will not be able to find replacement parts easilly.