There are 27 mana sources + Argivan Find which functions as an any color Dark Ritual so the 3rd mana is not too hard to find.
If you miss your turn 2 land drop you are usually in trouble either way.
If you have Fastbond + Crucible in hand then you can win with just the 2 mana you used for Oath.
If you really just don't have the win, then throw back Sol Ring and Black Lotus with your Memory's Journey and win on turn 3 instead of turn 2. As far as hands missing a turn 2 land drop, winning on turn 3 is pretty decent.
It is TERRIBLE to lose to one counterspell after Oathing and that is why there is a 2nd Memory's Journey.
If first Journey is countered, you have a second. The second one is very important and you brought up an excellent point!
I have even considered a 3rd Memory's Journey in the Spell Pierce slot. The graveyard hate is decent. Pitching to Force is decent. Occasionally I want to Flashback one Memory's Journey to use the other two to keep refilling my library so I never deck myself. Mostly this shows up if Coliseum was exiled and I have infinite life (which most decks just can't handle right now) so I just try to assemble a hand full of counters and wait for my opponent to deck themselves. This situation is rare enough that I consider the 3rd Memory's Journey not worth it.
So basically if you have 3 mana out and your Sevinne's Reclamation gets countered, then the next turn you can throw back Fastbond + Crucible + Argivian Find. You won't get them all in one turn, but it now takes a total of 3 Counter Spells to stop your win. I am not saying this is ideal, but it is better than just losing to a single counterspell.
I can't speak to what happens post board because I have not settled on a sideboard and I have not even settled on how to board against this deck. Is Grafdigger's Cage even worth it?
Pre-board against the top decks according to mtggoldfish the two main forms of maindeck hate are Ziasbond using Crop Rotation for Bojuka Bog, Survival having Deathrite Shaman and White Eldrazi having Thalia who makes my land be tapped.
Deathrite Shaman is a disaster, but the list on mtg goldfish only has 2 and usually you can race them if have to use Survival to find Shaman. Also you might have Mental Misstep or Force for Shaman. Thalia is scary, but occasionally too slow and there are no tutors to find Thalia, but she is a huge problem.
Crop Rotation for Bojuka Bog just beats me if I don't have the counter. One of the knocks on Spell Pierce is that Ziasbond is likely to have the extra 2 mana to ignore my Spell Pierce on Crop Rotation.
These three hate pieces are one of the main reasons that I badly need Force of Will and to a lesser extent Mental Misstep. Shaman and Thalia are huge knocks against Spell Pierce. Force of Negation won't work either.
I also tested Daze which is solid against Thalia, but frequently too slow against Shaman and is even less likely to work against Ziasbond.
Getting back to the counters. Of the top 8 decks on mtggoldfish only 2 even run counters. Those are Paradoxical and Dreadhorde. Paradoxical runs 4 relevant counters because Mental Misstep can't counter Reclamation or Memory's Journey. Paradoxical will need to use 3 of its 4 Force of Wills over 4 turns and hope that you have none of your counters. I am comfortable conceding the loss in a situation that rare.
Dreadhorde runs 6-9 relevant counters. 3 Pyroblasts can only counter the Memory's Journey which is why there is a second. This is probably what you had in mind, but it is still unlikely that Dread Horde can assemble 3 non Pyro counters over 4 turns or double Pyro in one turn or Pyro one turns and then 3 non pyro counters the next 3 turns. Imagine you are the Dreadhorde player and you have like 3 counters in hand. I am pretty sure you just counter Oath. Non need to risk some huge creature coming into play next turn.
Dreadehorde is actually a major motivation for adding the Oaths. Basically you can't keep up in card advantage against Dreadhorde and mana denial is ineffective. Oath is one of the best ways to deal with Dreadhorde. Ziasbond keeps its Dreadhorde matchup decent by running 0 blue spells and zero creatures so Swords, Bolt and Pyro lack targets. The lists that I have been testing all either had blue cards to Pyroblast or Ramunap Excavator or Collector Ouphe to bold or Swords. Oath is actually really good in that matchup!
So as far as maindeck hate goes I worry about Thalia, Crop Rotation for Bojuka Bog and Shaman far more than seeing 3 Force of Wills over 4 turns while having no counters myself.
If going a single creature route then another thought I had is Muldrotha the Gravetide.
She lets you play both Fastbond and Crucible out of the yard for a total of 4 mana. Since you can get one land from your yard to begin with you really only need 3 mana, so she is an alternative to Rasputin Dreamweaver and actually has some synergy with Ziasbond in general although her colors are wrong. I liked Rasputin in the Ziasbond variant because on problem is the Resistors can make it touch to combo out and Rasputin just dodges that because the 7 mana is plenty.
For my list Muldrotha has the benefit of being reasonable to hardcast (if I can find a black source) and being blue. Any creature that can be pitched to Force is huge because a limitation of my list is it only has 19 Blue cards so every blue card helps to keep Force viable.
I am still leaning towards creatureless in the maindeck because I just don't have room for multiple Stadiums or Coliseums and then creatures that ignore the graveyard post board since I will mostly want them when my yard is hated out.
Because of the maindeck hate I am tempted to add a Chain of Vapor in place of Spell Pierce. A singleton Chain helps a lot against Thalia. A main drawback is having fewer counters for Crop Rotation.