Sorry I haven't kept up but I'll answer all the questions in one post here..
A regular ferrule either needs to be TIG welded on or you CAN use silver solder if you pull a flare or "dimple" the hole so that you have increase surface area for the solder contact. This solder-flange version provides even more surface area for solder than the flare/dimple technique and you don't need to source parts for the dimpling tool so it's BETTER(tm)
I think just about anyone can do it. The biggest challenge is finding a cheap enough 1" conduit punch for the hole itself. I happened to have scored a nice Greenlee 4-size set on Ebay for $65, but I think they retail for over $100. The process for soldering the TC flange will very similar to the spuds, but use less solder due to the close contact between the pot and curved flange.
Correct. You'll solder the TC flange to the kettle first. Now, you can use the TC clamp and gasket to attach the element adapter (with element installed into it) to install the assembly into the TC flange. It's now an electric kettle. If you don't want to use the element for one brew, you remove the assembly and clamp a blank 1.5" TC cover on to seal the hole. The ONLY thing you need to be concerned with is dry firing. Once you have liquid in the pot, at least covering the TC flange area, there's no way it's going to heat over the silicone gasket's tolerance. They're good for 500F.
I think an OD threaded solution could work. I just haven't gotten there yet. I'm trying not to overload my manufacturer with prototype requests because I know it disrupts production runs on other parts (that I really can't wait for).