I really like this thread! Thanks for taking the time and doing all this work!

Since you have followed the conservative, practical suggestions of the guys here and it still does not shave well may I suggest something?

All of the used razors that I have purchased have shown the effects of time on the bevel. Water and oxygen play heck with the bevel. It is usually weakened from the oxidation. Usually I have to use a 1000 stone to remove the old, weakened steel and get to steel that has not been affected by the oxidation and establish a new edge. If I don't then the edge simply goes to hell at some stage in the honing process. Either the bevel develops small chips at the finer grits levels or it simply will not develop an edge because the edge is soft from the oxidation and crumbles making it somewhat "rounded" and rough.

My point? Well, you have done all the proper steps. Now its my guess, and only a guess, that you need to wash you blade with soap and water then strop it on a terry cloth towel. This will remove all the "stuff" from the blade.
Then take a pic and perform the thumbnail test. Next, go back to the 1000 grit stone for 25 laps. Clean the razor as above and take anothe pic and perform the tests. Hopefully, you will see a distinct change in the bevel and feel it in your tests. If you are sure that you now have "new" steel to create a bevel on then move up the grit ladder as normal.

Just my two cents,