I think the prime reason why I didn't put priest and wiz higher is how their specific tricks are reliant on specific spell level. So often you'll end up saving slots, or having to choose between Miasma and Mirror Images, etc...
Druid less so, since their 2 main uses (DoT and HoT) have better redundancy between tiers.
This problem is fixed by Bloodmage though.
Note that the gaps between tiers is thin (exception with the 3 "bad SC), that's why not taking Subclasses into account is valid : I'm only trying to make a synthesis, you have "great" builds for each class.
Engagement slots can be found elsewhere : barbarian shout, modals, items (Kapana Taga). Fighters have undoubtly the most convenient. But you can build almost as good tanks with added utility with other classes.
That said, fighters have their own tricks : Tactician, Devoted PEN, quickswitches, access to a multi-hit weapon attack as MC... Though not as class defining, they enable perfectly strong builds. "Solid" tier simply means you have to think a little more to make these classes optimal.
Nice !
By the way, sorry if I don't answer all your points (I don't have that much time)