Very nice and detailed set of posts.
I agree that Griselbrand is by far the worst card in the deck. He is almost entirely uncastable and more frequently than ever, late to the party, whether that is an overpopulated Shops board or some combination of Jace across the table, low life, or alternative situations of opponents' overdevelopment. I cringe every time I draw him and feel like I want to cut him forever. His main roles are:
Being the general purpose "best creature" across the board g1 after activating your Oath, which is something more likely to happen in a g1 than any other game in the match.
Being relevant against Dredge and Combo, which are the two match-ups where Inferno Titan most often falls short.
However, he is not indespensible. In 2016, I played Dromoka/Auriok Gush Oath at Champs without Griselbrand and placed in 17th and IIRC it was the highest placing Oath deck that year. I don't think I ever missed him and I probably benefited from never drawing that horrendously dead card. Side note: has anyone noticed that MTGO seems to put Griselbrand in your hand far more often than it should? There's no such thing as a pure 100% randomizer, though some experts can come close, but it would not surprise me if MTGO's falls very far from optimal benchmarks or even contains some metrics to influence the game-play experience that they believe would somehow increase the program's profitability.
Other general thoughts:
Griselbrand is slightly more feasible in builds that are maindecking Underground Sea but running black may or may not be desirable at that given point in time.
Boarding out a lot of Oath material is indeed a proper strategy in post-sideboard games, particularly against Landstill where Forbidden Orchard is outright dreadful.
Arlinn is very strong in multiple contexts including UW Landstill which you mention. Dack Fayden is the best planeswalker in the list and then it's a toss-up between Jace and Arlinn. I've gone down to 1 Jace before several times without issues. It's often not worth slithering around Pyroblast (or failing to do so) to put an imperiled planeswalker on a board with Spirit tokens and Oath is not like the hard-control decks that really abuse Jace. Chandra is a good planeswalker in general though she's also not good at protecting herself from Spirits and access to RR is a big barrier, since the fetch protocol for Oath wants you to have at least one green and at least one red before trying to double up on one of the secondary colors (if that's even possible, with disgusting Wastelands being so rampant). I would rate Xenagos and Nahiri (if you have white) right behind Jace as preferable planeswalkers to Chandra in modern Oath builds. For those reasons, having a second Arlinn or additional non-blue planeswalker is not a bad call.
Finally, the Punishing Fire take on Oath of Druids has its charms though the lists I've seen have suffered from only being able to support a few Moxen (weakening Thorn matches) and they are much softer to RItual, Dredge, Paradoxical combo, and Karakas than Inferno lists.
Good luck with your refining, -B