Don't forget, almost all of the Grand classes have multiple candidates for who can fill those roles; presumably, different Heroic Spirits would be summoned as Grand Servants to deal with different Beasts. The only exception being Grand Assassin, as King Hassan is the only one who qualifies for that role. So if you want someone other than Noah as Grand Rider, that's just fine.
indeed. My reasoning for Qin Shi Huang was Sherlock mentioned his LB versions Saint graph was Grand Quality in spite of being a Ruler, and Between getting traded out for Iskander in Zero and almost being False rider in Strange Fake, i connected the two.
Admittedly, i'm not sure who could fill grand assassin. the closest i have are Shiki Ryougi (Death perception) and False Assassin and her recreation of the first 18 Zabanya's.