Honestly the only thing I agree with is Dissipation needing a change, and as everyone has said, making it so it would make Summon or Summon II instant and MP-free would be perfection.
Everything else you said is highly questionable.
SCH has such high MP costs because 60 to 70% of our potential HPS is literally completely free. Even more so now with Fey Union. Also, the new Aetherflow trait makes it so SCH can potentially ignore Lucid Dreaming if they're feeling ballsy. I personally wouldn't, but my playstyle is now very MP intensive. This doesn't mean MP is a problem for me, however. Specially not now with QA. Adlo and Succor could use a slight MP cost reduction, but I no longer think it's absolutely necessary or that it is too detrimental to the job. If anything, it cements its designed weaknesses further, which isn't necessarily a bad thing.
Fairy AI is clunky? Yes, it is, but this never stopped the job from having a guaranteed spot throughout all of HW, and it won't drag it down now. A good SCH will always find a way around pet AI after all.
Even without the Fey Union DH bug, it's actually strong enough to be the only healing source you need going out during a double trash pull. A triple trash pull might need you to use WD before going for Union, but the ticks are pretty strong. It won't keep the tank topped off, but the steady healing is enough to let you fully DPS through an entire pull if you build the gauge enough. The gauge drop-off rate is also super slow, and Quickened Aetherflow makes it so it charges much faster. I agree it takes a very long time to activate, though, but again, see my point Fairy AI.
Whenever Excog doesn't proc for me, it means I used it wrong. Learn to know when each part of your kit is the better option. An 800 potency heal isn't something that you should be able to throw around freely, specially not on a 45s CD and with another 600 potency option on a 1s CD. This is like throwing out Indomitability when you could have safely used Whispering Dawn instead. Waste of situational resources is the player's fault, not the tool's.
Miasma II is a close second to Holy for "strongest healer aoe DPS spell." Gravity is hot trash in comparison to both. If anything, Miasma II's range is its only weakness. Everything else about it is superior to its counterparts: more damage, less MP, instant (and shorter cast time compared to 3s for the others), more versatility due to its DoT, no damage scaling, and even good single target uses thanks to Energy Drain. I only put it second to Holy because the stun Holy brings is a massive form of mitigation in trash.
Please don't take this as an attack or anything. I don't want to dismiss your opinion completely, but I think the SCH complains, particularly the ones you posted, are an overexaggeration. The job is actually on a rather comfortable spot right now.
My only true gripes with SCH right now are that Dissipation remains unchanged and that Selene is still literally just Fey Wind pre-pull, but these problems were very present in HW. Kind of a shame they're still not quite there.