Man that's a great list! I run a Scarab God too, but it's super budget by comparison. Love him as a commander though.
Personally I would look to cut
Graf Harvest. It would depend on your strategy, but personally I think Scarab God's zombies don't need to do all that much swinging. His drain and scry effects do enough work.
Lord of the Accursed can give you the same effect, and if you're going to pay 4 mana to
exile a creature from your graveyard and make a Zombie, you're going to want to use
the Scarab God's ability rather than that of
Graf Harvest.
I think fundamentally though it would depend on your strategy. Are you looking more at a horde, "more zombies = more good",
flood the board kind of thing or more of a control, let
the Scarab god do the work kind of thing?
For the first, I'd keep
Dark Salvation and git rid of
Dread summons, for the latter I'd do the opposite. A huge thing with
the Scarab God is that he can target opponent's graveyards, so if you just want him on board to
dominate then
dread summons is great at providing options for his reanimation. If it's more about making a lot of Zombies and Scarab God's upkeep trigger is too delicious to ignore, then
Dark Salvation is the better keep for me.