Kill Othinus? That was never stated, unless you're talking about a depowered Othinus that's literally dying because of the Fairy Stake.
Othinus not destroying the universe because of her presence? That's probably because she can perfectly control her powers. It's stated to be infinite multiple times yet the power of Phase Beings summoned by Dainsleif is explicitly finite, and anything that reaches actual infinity is infinitely greater than something finite.
You're also ignoring that one of the calamities was Surtr who is supposed to be weaker than Odin in their theology without breaking his destiny. Othinus is Odin.
I don't see any proof that Othinus was weaker than they were.
It's not referring to a depowered Othinus.
Othinus directly says that the weapon is appropriate to oppose "a Magic God".
Not a weakened Magic God, a Magic God period.
Why tf would she be referring to her depowered self?
The following sentence only says she could beat Marian and not let her summon said calamities in the first place, not that she's stronger than them.
"If Othinus had been at full strength, she could have crushed Marian in a single blow.
There would not even be a need to kill her. She could simply throw her into a world of happiness."
There's no need to fight the calamities in a normal fight, she can just phase shift gg Marian and win.
They're not stated to be finite last I checked, and even then it doesn't matter.
"The power refined to kill a Magic God was being turned toward a mere boy, yet she could not stop it."
That's all well and good.. but I don't see how this part is relevant. Othinus isn't Odin the phase being.
And I never said she was weaker. I said they were comparable.