disdainful stroke is far more limiting than
disallow so the extra U mana is worth it.
you could also run
Triskelion and go infinite with Makius the unhallowed.
The thing is you don't really want to be sacrificing your zombies because you want as many as you can possibly get and keep on the field for
The Scarab God to really do his thing and
deal damage (also I take back what I said about
mindcrank. Definitely keep it). The big problem with blue black zombie decks is that they take a while to swarm and build up their army because they don't really have any token multipliers like
parallel lives or
anointed procession. Don't get me wrong they can build up fast with the right set up, but one board wipe and it can take a while to get them back. so unless you're getting something from sacrificing them (recursion, infinite mana/damage, milling) there isn't really any point in running cards that sac them.
As for
Prismatic Lens and
fellwar stone, you should definetly replace them with
mana geode and
commander's sphere. They are both cheap cards (pretty sure commander sphere is in virtually every commander precon ever printed) and
mana geode is a common from war of the spark (I think...which was the most recent booster set printed I believe) so neither of them should cost very much. I say that because
Fellwar stone is just not very good outside of 3+color decks because it relies on your opponen'ts lands, and while black and blue are popular colors, there is no gurantee you'll get one or both of them in any given game and
prismatic lens is just garbage. Yeah it lets you "fix" your mana, at the cost of 1 mana and lets you scry upon
entering, but
Mana Geode lets you scy and taps for one of any color without the need to pay that 1 generic mana to do it and its a common valued at 25 cents and
commander's sphere is about $2. well worth it to make your mana base more stable.
I should have mentioned this earlier, but most commander decks are built around their commander and generally have a strategy that they fully commit to, but having a back up is always good. Though if you have too many back ups then your deck will end up being pretty dysfunctional. So if you want to make your wincon passive damage via
the scarab god you'll want to focus on token production and not sacrificing them without a reason (like the
gravecrawler/phyrixian altar/diargraf captain combo) which also gives you the option of just running opponents over with a horde/army of zombies and not worry about big mana sinks like
torment of hailfire (even though it is a really good card and you should pick it up just on principle if you can afford it even if you don't use it in this deck). If you want to go the sacrifice for value route then I would suggest adding
blood artist and
zulaport cutthroat into the deck even though they aren't zombies they are combo pieces that will let you passively kill opponets by sacrficing or killing large numbers of creatures. Just keep in mind you'll want to maintain that large army of zombies for
the scarab god in either situation unless you're going in for the kill.