You listed a bunch of changes suggested by this thread
I thought you were the same guy.
For dispersion it makes sense to be baseline, utility has been removed from many spec trees and it should be removed from the shadow spec tree.
Door of shadows is an easy win, its not an overpowered skill on its own in fact its a bit of a clunky slow mobility spell that without any of the buffs is more than acceptable to give to shadow priests.
Spectral guise was awesome, i will always advocate adding it back into the game, and dont just make it a pvp talent that skill had so many uses in regular content it should be a core part of shadows utility, id trade almost anything for it back.
Its a shame idol isnt very good, mind bender with inescapable torment is my favorite cooldown, i really enjoyed trying to extend it as much as possible with SW:D and mind blast and seeing the explostion felt very good. Its the kind of gameplay void eruption tries to achieve but fails at and its far more interesting than dark ascension, id gladly let those other two CDs get deleted in favor of mind bender replacing them in the center where idol is more flavor than a capstone.
Void torrent being central is good, no argument there.
Im going to suggest this every opportunity, make shadow crash (with whispering shadows) baseline but exchange its CD for a 50 insanity cost, alongside a passive that sets insanity to 50 out of combat like natures balance that balance druids get. Tying 100% of our aoe to a 20s CD is ridiculous.
Honestly i think they should just delete mind spike, its a garbage spell, id rather they put deathspeaker in its place. At most they could make it replace mind flay during dark ascension since DA doesnt affect mind flay for some bizarre reason. If they do keep it as an option it needs more supporting talents not less, something to really drive its gameplay in a unique direction, not just filling the exact same role as mind flay but cast time instead of a channel. Even a passive option that made mind flay apply mind spikes to the target that are detonated by mind blast would be more interesting then replacing a weak filler with a slightly less weak filler.
Mindgames was a cool spell but a garbage addition to the rotation as shadow has enough plates to spin, and it added nothing more to the rotation. Its more a problem with it not being optional and the tree design forcing players to take every active you can reach. Blizz really needs to utilize the talent tree better by making themed branches with optional skills at the top that are weak alone but gain more power as you spec into them so they are truly optional.