(1) Hidden as in they are there, but client doesn't show them.
(2) Uh, +healcrit and +damagecrit are 2 benefits, so plural seems right here.
(3) I wondered about this as well, INT and WIS do not benefit from the hidden strength AA and thus have no hidden cap. I figured it was just a bad formulation.
(4) No stat does the same it did pre hidden cap post hidden cap. So why would you expect INT/WIS to be different?
(5) Again, bad formulation. Essentially this tome gives you a 20 points hidden cap per rank of the tome. To get the actual benefits of the hidden cap, you obviously need a high enough ability score. So if you have an INT/WIS of <random high number> but only done Tome of the Mind I, only 20 of these points help you. Once you have finished Tome of the Mind II, another 20 will help you, for a total of 4o points. Any points above what is covered by tomes have no effect at all.
(6) Cap for INT/WIS is 355 after doing all AAs to raise it. All other stats cap at 305, but are affected by hidden strength AA, wich raises their hidden cap to 485.
(7) Theoretically yes, but good luck on amassing that much WIS on an INTcaster.