Sorry my post is going to be a bit negative in general, just want to forewarn you, but I am more helpful at the end. I just need to establish what and why I don't think a lot of things work that well.
The reanimation cards in
Apprentice Necromancer,
Feldon of the Third Path,
Corpse Dance seem very weak. You only have
Court of Cunning to turn your graveyard into a resource and your reanimation targets are pretty limited and really don't add much to the game if you do keep the copies around.
You might as well just have
Animate Dead,
Dance of the Dead,
Necromancy in those spots as Apprentice and Feldon do have summoning sickness so are slow and easy to predict what you are going to try and require investing more mana .
I suppose using
Feldon of the Third Path or
Corpse Dance specifically on
Snapcaster Mage or
Spellseeker each turn could give you advantages, but investing
2r for Feldon and
4b for
Corpse Dance and then expected to have more mana for casting the Snapcaster or Spellseeker targets in cEDH doesn't seem very likely.
Then actually keeping those creatures in play doesn't effect the game at all.
Tainted Pact is usually mean't to be a
Demonic Consultation alternative but you literally have to have no two cards with the same name in order to exile your whole deck and you are running multiple Island and Swamp.
The best you should do is to run 1 x
Island, 1 x
Snow-Covered Island, 1 x
Swamp, 1 x
Snow-Covered Swamp.
The
Dramatic Reversal with
Isochron Scepter is looking super weak in this deck.
Chrome Mox,
Mana Crypt,
Mox Diamond,
Sol Ring are the only sources of nonland mana.
Final Fortune is the only extra turn card you can match up with
Isochron Scepter. This is the crown jewel to running Obeka but I just want to point out you can only imprint instants.
If you are matching up
Discontinuity with say
Warrior's Oath,
Last Chance,
Final Fortune then it is a two card combo to get a single extra turn. The deck only has two cards that have static card advantages in
Dark Confidant and
Court of Cunning. My point is that the card doesn't have a lot of uses.
Twinflame just like the reanimation cards seems a bit of a wasted slot. I suppose having 2 x Hullbreacher or Opposition Agent gives you more resilience to targeted removal. But my point is that there are just so few targets for the card that it can just end up being a do nothing spell and terrible top deck.
I have made an
Obeka, Brute Chronologist deck that is more aimed at doing over the top big things so is more casual.
But one idea I do think that you can look to abuse for cEDH is
Underworld Breach with the extra turn cards. Because Obeka can end the turn it will stop the trigger on
Underworld Breach from removing itself. With the extra turn cards, you will be the next player so this means it will remain in play.
The idea is that as long as you have 3 cards to exile in graveyard, you can keep taking extra turns via the
Underworld Breach staying in play constantly.
Therefore I think you should look to set this up as more of an end goal.
Brain Freeze and
Lion's Eye Diamond are some of the main sources of fueling
Underworld Breach.
Court of Cunning is a good one for you. If you have
Underworld Breach in hand, there is a high likelihood that you'll mill an extra turn card and then because the Court mills 10 cards you can go infinite.
Therefore I think some number of other big mills cards could be the plan. But I can't stress how you need
Underworld Breach in hand because Grixis doesn't have a lot of ways to get Enchantment cards from your graveyard. What I mean is that you can't just jam mill cards onto the board until you have
Underworld Breach.
Mesmeric Orb,
Ashiok, Dream Render,
Nephalia Drownyard are the ones I'd play.
Obeka +
Nephalia Drownyard +
Underworld Breach + extra turn card = infinite turns. That is actually pretty feasible.
Ashiok, Dream Render has a limited number of uses, but hopefully should mill into something over 4 activation's (or 5) that allow you to continue, plus obviously it's great in cEDH at stopping searching.
It is possible that you run the extra turn cards in
Capture of Jingzhou,
Temporal Manipulation,
Time Warp as well as it is still the net result that as long as you can keep exiling 3 cards from your graveyard you can keep casting those extra turn cards with
Underworld Breach and Obeka.
Intuition is a supplement card to
Underworld Breach as you can search for say
Brain Freeze,
Lion's Eye Diamond,
Warrior's Oath for example which will end up with a win (if you have Breach).
If you are playing all those extra turn cards I mentioned (which I think you should) then it will be nice to have a few more static card advantages. That is ways to draw cards or whatever with additional turns.
Planeswalkers are often the best for this.
Dack Fayden and
Jace, Wielder of Mysteries puts you very close to putting 3 cards in your graveyard for the Breach/extra turn combo. If you are just casting one other spell each turn (besides the extra turn) then this will be good enough which is HIGHLY likely.
Jace, Wielder of Mysteries is also a win con with
Demonic Consultation/
Tainted Pact.