No ones asking for limited jobs to be removed here fyi, so a dungeonable version would not be "gutted" as much as balanced whist the limit aspect remains for those who enjoy that.
Yes another trick wouldnt be a problem, buff stacking has existed as long as the game has
Again with "gutting". Im of the fairly controversial take of white wind would not be broken in raids (provided they put in something to account for health snapshotting) because it would almost always be more beneficial to have another healer cover the damage outside of niche settings, however I dont think a more balanced white wind would be "gutting" it either, pld and rdm and war all have access to fairly potent heals too.
Double Regen comp isn't used for prog often, and if its farm then you wouldnt make a dps waste gcds on shielding for the healers when one could just play noct
Diamondback isnt an immunity, and actually wouldnt work on a lot of the tank busters that happen these days, like stone crusher ect due to them dealing literally millions of damage if you dont swap/immunity, and unless youre wasting further gcds on froglegs and such there are very few mechanics that can be invulned without taking aggro that can't also be done with a shielded mnk if you're so determined.
Personally im fine with blu being the lowest dps in the game if it meant i could raid on it, but utility tax needn't be that harsh, you also seem to have a very loose definition of "gutted", a lot of blu's stuff could work well in a slightly nerfed setting, which wouldnt take away from the jobs identity just by the nature of them being nerfed, and if you wanted the un nerfed versions just play limited content, again we are not advocating for the removal of that.