When using the War Caster feat and an Echo Knight Fighter’s Manifest Echo where does the spell originate from?

Inspired by the following question:

  • Can an Echo Knight with the War Caster feat use cantrips for opportunity attacks made by their Echo?

The Echo Knight’s Manifest Echo feature states (emphasis mine):

[…] When a creature that you can see within 5 feet of your echo moves at least 5 feet away from it, you can use your reaction to make an opportunity attack against that creature as if you were in the echo’s space. […]

And the War Caster feat states (emphasis mine):

[…] When a hostile creature’s movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. […]

I am aware that the spell cast using War Caster does not count as an Opportunity Attack. And this makes me unsure what happens when a creature provokes an Opportunity Attack from the Echo and the Fighter chooses to use War Caster.

Is the spell cast from the Echo’s space because the Opportunity Attack would have been made from that space?
Is the spell cast from the Fighter’s space because the Maifest Echo feature only allows Opportunity Attacks to be made from the Echo’s space and not spells?
Is something else the case?