There are reasons for each of them, but they may not be useful enough to keep. Gravity Well is one of the few defenses against fliers the deck has. Quest for Renewal allows me to untap things like Patron of Orochi multiple times a round (that one could go though) Elixir of Immortality is really there just as a way to recycle the graveyard. I used to play against a lot of mill happy decks, but games rarely last long enough to make it worth it for the most part. Predator Ooze was just in there for its ability to be an immortal blocker and quickly grow into something more useful. Oran-Rief Hydra is a bit weak on the first turn it is played, but quickly grows into something scary gaining +2/+2 every turn or so. Panharmonicon is an odd one as it has a reason for being there. It works well with several other cards (Acidic Slime, Avenger of Zendikar, Eternal Witness, Fierce Empath, Kalonian Twingrove, pelakka wurm, terastodon, tornado elemental, ulvenwald hydra, wood elves) and then it can also combine with the Conjuror's Closet to bounce any of those creatures, triggering the ETB effect again.

34 lands is a bit light, and I will agree that sometimes it does leave me hanging, but there are so many other mana sources, ways to get more land, mana multipliers, and other things that most of the time it is more than enough to get things rolling. I played around with more lands and more mana ramp in various ways, but it never seemed to make enough off a difference to be worth it in the end.

I love Castle Garenbrig. I stopped buying newer sets so I haven't seen that one before and will definitely be looking to pick one of those up.