You'd have to give a citation on that because the current SCH has no resemblance of HoT Healing (Which was given to WHM again who has emphasis on hp restoration, which breaks down into HoTs). And even then it'd all have to come into place thematically and composition wise (WHM already fills that meta, unless you want them to be the shield oriented healer instead of a job whose lore is centered around planning ahead and military tactics). I'd also take everything that yoshida says with a grain of salt, since not all of them either comes to pass or as we expected (such as MCH ammo usage and the two ranged classes in general).
Lustrate is the level 50 job skill. Shadowflare is a level 50 class skill, sacred soil is a level 45 job skill. This sort of approach would take a lot more work on a job whose already functioning well in the current set up.
You still would. Your DPS relative to the DPS would just be less.
Job crystals in a sense act as the "second half" of the class that it's being added onto in the way they add onto it. Like you said, it'd require changes to the class abilities to make each job crystal more feasible, but if we're going that route, is it necessary to force a job into that soul crystal anyway compared to giving them a new slate to work with? In regards to scholar there's a plethora of redesigning and balancing just to make it work (and in this case, for the sake of doing so since SCHs are pretty well off at the moment)
That'd actually sounds really really boring from a tank perspective outside of a raid environment. Not to mention if the enemies are that strong, then dps/healers would die if they even got breathed on.
The end question is what sort of role do you want the job crystals to be. I see it as something that just adds onto the class to complete it, rather than a "promotion" system of sorts that some MMOs have. Branching would work a lot better if it was done eariler in the levleing process (at say, 15) to diverse the abilities some more, rather than changing existing skills this late into the game.