That flat ~200 potency stronger shield is definitely more favorable than the heal it brings (along with it costing less mp and being instant cast). You don't use the shields for the healing. You have healing spells and abilities for that. As for Adlo being stronger, it is only stronger during a crit and those only happens ~15% of the time and having to fish for it will leave you oom, then you have to get everyone stacked near the guy that it popped on. All that for only a 100 potency stronger shield. Succor is roughly 25% more mana, and mitigates for less as well. Again, Noct is better at this shield as well (but only by about 50 potency).
I believe you may be downplaying the cards a bit, too. Giving a raid-wide 5% damage buff brings in 40% more damage than your team could pull otherwise. Even 10% on the right person is pretty good. Or even 10% less damage on everyone is pretty good, and 20% less on the tank is like giving them a free Bulwark that lasts longer than the real thing.
I definitely agree with you on the Whm being in a perfect spot. When I go to solo heal Susano, Whm is the only one I would take, since I run with PUGs and Whm can fix the "happy little accidents" much more easily than the other two.
I agree to an extent with "Many scholars complain about the slightly lower potency, but sch have three spells that can boost their healing potency(AST only have one) and they have the strongest 0GCD and free powerful abilities among all healers". They have powerful aetherflow abilities, but they are to planned accordingly and carefully. If you reach a point where your group has made two high damage mistakes, you better hope they don't make a third because you don't have Helios or Medica to back up the Indom and ET you had to use. I am perfectly fine with us having lower potency heals than the other two. I never had complaints until this x-pac, and my biggest is that we are more throughput healers than ever here.
As for "Sch's identity is not the shields(Not anymore since there are three healers for two positions in party)", that shouldn't be a problem. There are so many MMOs with shield healers and then have 5-10 other healer types as well, that if this is a problem, then it is a failure on the devs end.