Wow, what a spectacular post. Thank you for sharing all of your insight here.
1. Null Rods in the main deck
I agree with you about Null Rod. I went to the full four in the maindeck recently and shoved some of the Spheres to the sideboard. I like having at least a couple of Spheres in the maindeck in the Workshop style list.
I like that a Sphere and a Null Rod can partner up to do a decent Defense Grid impression under the right circumstances, which is critical with all of those Force of Vigors lurking about.
2. Reactive Countermagic and Permanents
I've also been unimpressed with Pyroblast, Mindbreak Trap, and Leyline of Sanctity. Both are too situational and get in the way of developing the board.
However, I think that [[Veil of Summer]] has promise, as it's more flexible and replaces itself. I tried it in a few leagues last night and I was impressed. The card was responsible for winning several games/matches against PO and one against storm. I don't think anything else would have worked in its place.
[[Veil of Summer]] counters the following (and draws a card in the process):
a. bounce effects: Hurkyl's Recall, Chain of Vapor
b. discard: Thoughtseize, Unmask, Cabal Therapy
c. removal: Assassin's Trophy, Abrupt Decay
d. graveyard hate: Ravenous Trap
e. blue counterspells: even Flusterstorm!
f. permanent-based exile/bounce: Ashen Rider's enter-the-battlefield triggers, JTMS,
Teferi(Edit: nope, can't instant speed)
All that said, it wouldn't surprise me if Thoughtseize is still the better card to play. It does much of the same job and has more flexibility.
3. Oath of Druids
I have mixed feelings about Oath.
The card is there as much for "tutoring" Loam as it is for summoning the big fellas. The idea is that Oath forces them to have two kinds of answers. If they have Priest/Cage, you get to use Loam if they have no graveyard hate. That has come up several times. If they have graveyard hate but nothing for Oath, well, you get the idea. The best target, by far, has been [[Muldrotha, the Gravetide]]. You get to play Crucible and Fastbond from the yard and go off.
Another thing I like about Oath is that it can be used to increase the range of acceptable hands. For example, if I am against combo or storm, I want a lockpiece like Null Rod. However, a hand with Null Rod and no reliable path to victory won't get there. Increasing the number of plausible paths to victory likewise increases the probability of keeping a decent hand with Null Rod.
Oath does have problems, though:
a. It's best in the creature-based matchups (Shops, Dredge, and Survival). Obviously, this is not a huge selling point, and necessarily takes away from cards that are better against our "problem children."
b. Hard to cast the big creatures, giving blank draws
c. Makes their Force of Vigors even better
d. Waters down the traditional lands package (you have to cut somewhere, and it's generally not the disruption)
Some of these issues can be mitigated by the choice of Oath package. Using castable creatures is a big one. I've Oathed into Ramunap Excavator. Courser of Kruphix seems plenty reasonable, too.
I've also Oathed into Collector Ouphe and...it wasn't bad given that the Ouphe comes in only in the matchups where it's very impactful.
I've also tried cards like Draglonlord Dromoka to shut off Ravenous Trap etc to get at least one Loam activation.
There's a lot to try with Oath.
Anyway, you've also mentioned a bunch of other possibilities. I'll be mulling these over and testing them.
Thanks again for sharing your thoughts. Good luck with the brewing!