Must...climb...wall of text...must...organize....thoughts!
1) Back End - I don't see any problem with this at all. My ability to tank depends on the SCH's ability to keep me alive, a Dragoon's dps depends on my ability to keep Enmity, it would make the game more fun if more collaboration was needed. This seems like something we disagree on in a non-mechanically fashion. You said "Performance being solely contingent upon the play of others" (paraphrasing)...isn't that the definition of a support role...? Am I crazy? It sounds like you don't like support roles, lol.
2) Solo Play - Every class has some dps abilities to it, even CNJ, you can easily create a cleric-stance like ability to compensate for this. My vision of Demi would do that just fine. Fire potency "Y" would turn into Fire potency "Y"x2 within demi's circle if you are soloing. For Dancer...I dunno give it the stance "Break Dance" where auto attack potency is tripled, couple that with pure dps songs playing and you get competitive solo-style play with any other class. I don't see an issue with this at all. Easy to fix. Next.
3) % based buff/debuffs: Uhhh....Rage of Halone, Butcher's Block, Haymaker, Fight or Flight, Featherfoot, Blood for Blood, Mantra . . . all of those are % based, so its fine. I get your point of gear needing to make a different in the main aspect of the job, so fine, make them hard wired to weapon value, I don't care. Its completely irrelevant to the main point.
4) Multiple support characters: What? If you ran 6 support chars, 1 tank, 1 healer, who would do the dps? It sounds like they just made the support chars OPed in the game you were talking about. That argument has no substance, I feel like you are just trolling now. I have played several MMOs with *explicit* support characters, and stacking them together didn't do anything because it didn't offer enough benefit. You have literally no ground to stand on with this. Everquest 1 has 2 (arguably 3) *explicit* support roles, and has been balanced and around for 13 years, and those classes are not stacked in raids.
5) On your last paragraph that I quoted, I suspect you are right. It would certainly be possible and fun, but they probably won't do it. I will concede that it would be problematic if you used DF and got DNC DNC WAR SCH for a dungeon, or TMM TMM PLD WHM