New to Grendel, scope and scope mount question

[Klem]

I am also uncertain about the cantilever requirement when everyone and their mother are running full length picatinny rails on their handguards

I don't trust the stability of a handguard for long-range magnified optics. It's a popular opinion which is why the cantilever market exists. If you're thinking of straddling the join with a clamping mount, as opposed to two separate mounts then that will probably work too.