"If they had only a small stash of purloined or purchased powder, all the more reason for them not to waste it on some 2-bit attack. Unless it wasn't the 2-bit attack that it seems, and there was some concreet reason for it."
2-bit is your expectation.
"Senators? Or flash-in-the-pan pop-stars? Hm, that is a hard one."
Senators, yes, but that was the second wave after the first didn't work. "Flash in the Pan". You might not be familiar with J.Lo., or overly influenced by her post-9/11 Gigli debacle. J.Lo. was a huge movie star and singer, well, "music personality." And she was even bigger overseas, her albums selling more there, especially Latin America, Middle East and maybe also Europe. She was the biggest American entertainment personality in the world - if not #1 in America, though close to that.
With marriages and Gigli her star has declined.
"For geographical proximity, you can't beat American Media. Was not it almost next door to an airfield the highjackers flew at?"
Not sure how close, but Stevens lived close to it. BTW, I don't discount your thesis and find it interesting. In furtherance, how about the Gloria Irish connection? She was the estate agent married to an "editor" at the Sun or other AMI publication. Was this fact reported between 9/11 and 9/18? If so, in your frame job scenario, this could be the source of the targeting...or maybe it was known by others even if not reported in the media...
"Planted evidence not in the mailer's own handwriting would be advantageous, no?"
No rational criminal, a ransom-kidnapper, whatever, does not disguise their handwriting if they think it will lead to them (wasn't the old style to cut out individual letters from magazines?). But you assume it's not the mailer's own handwriting - why? And why would a "framer" write the sets of letters with different determination? Could a framer fake the consistencies between the two sets?
"Who else ??"
How about J.Lo.? As we learned these tabloids get all sorts of fan mail, the fans assuming it will be forwarded to the stars.
I believe the "AMI" letters were different than the others. No one remembers a text similar to the others. And there may have been two letters. Also the dispersal pattern is strange, as if the letter(s) were pre-opened and carted thereafter through the halls, not opened by the addressee or his assistant in proximity.
I think I remember that CDC (?) report might have explained something about this.