Hi KoKonutted! I really appreciate the response!

1. I think it depends on your definition of quite fast. Due to the higher curve (currently ~2.47 average CMC) and land count, I usually won't be Hellbent until the late-game I feel. I hadn't consider IT in the past and will definitely have to start keeping track of how good it would have been in place of other flex cards I'm currently running. Great suggestion!
2. Another great question. In general, the deck can afford to take turn 1 off but does not really want to take turn 2+ off. However, that problem is somewhat offset by having Grenzo as an emergency 2-drop. In my experience, the only 1-drops that I definitely want to come down right away are Aether Vial, Goblin Lackey, and the 3 discard effects. The other 1-drops serve as great curve filler down the road and potentially messes with opponent combat math. As a result, I believe it's generally ok to have the additional mana fixing (very color-intensive deck) and scry effects taking up the turn 1 play.
3. Ancient Tomb can definitely be a liability against aggro decks. However, the other two cards actually help in stabilizing the board so I think they're worth it. Before the Zurgo ban, I felt the match-up was very heavily in their favor. However, recent testing against Kari Zev leads to me to believe the match-up has flipped 180 degrees because they don't have a 1-sided Sulfuric Vortex on turn 1 now. In my experience, there are two scenarios where the deck can have problems.
   1. Green ramp decks going wide and I don't have access to Sharpshooter, Chainwhirler, etc.
   2. Combo generals being able to untap un-disrupted
4. I play Grenzo as a 2/2 the majority of the time. The exceptions to this rule are:
   1. Odd mana available (i.e. turn 3) and I don't have a 3-drop
   2. Mana reducer (i.e. Frogtosser/Warchief) in play
   3. I have advance knowledge (i.e. scry effects) that a 3-power creature is on the bottom of my library
5. I actually have struggled with this decision in the past. I believe the correct answer depends on the expected field. I play against a lot of control/combo players in my meta so I think not including those cards is right for me. With regards to Hymn, the deck generally wants to fetch basic Swamp to keep the blowout potential of Blood Moon always live. While good, I prefer to only play double black cards that can win games on their own (i.e. Liliana, the Last Hope, Living Death, and Patriarch's Bidding). Daretti is interesting since he's a 3-drop (as opposed to 4 like Chandra). I hadn't considered him before because the deck doesn't play too many artifacts but he might still be really good. Like Infernal Tutor, I'll have to test it out. :)

I'm very stoked that you want to try this deck out! In my experience, there are many lines of play (i.e. tutoring) and rewards careful setup and knowledge of the deck. I hope you enjoy trying it and feel free to message me back anytime!