@Yzarc, a question before I finalise my proposal, in the Wound where we're going, is the Umbra separate from the material world? If we wanted, for example, to go and find the Worldsoul while it's rising from the dead so we could directly interact with it, could we do that without buying Umbral Passage + Hand from Beyond?
Hmm... if we could have Fan cast SSC on the opposing Gnosis roll that might just work. Do all the Ghosts count as one spirit though?
They probably don't - but this is probably something you do after you've restrained or defeated a bunch of the ghosts.
By for example, 'killing' them with Ultimate Darkness Internalisation active and capturing them in Fan's anima:
When a target is killed, a spirit slaying charm is required for immortals, the Infernal can instead imprison those that they kill in their Anima or their inner world if they have it. Any being killed in such a manner is functionally dead and now a part of the Infernal's Anima or Inner World. They cannot escape and only by someone completely altering them, say by a Sorcerous ritual, or if the Infernal allows it, can they escape. Otherwise, they are completely at the mercy of the Infernal and cannot harm the Infernal in any way shape or form and can only die when the Infernal dies.
After engagements in which we make sure to have Fan perform the final blow, he'll accumulate ghosts and possibly corrupted spirits (fragments of the world soul) captured in his anima. At this point they can only escape if 'someone' completely alters them, or if the Infernal lets them escape.
That means that other 'someones' can affect captured entity (presumably with Fan's permission). It's at that point that Thalassa performs her spiritual surgery and releases the spirit from bondage, transforming the ghost into a spirit, with, as you say Fan helping with Shadow Spite Curse. If it works, Fan can then release the ex-ghost*, and if it fails he just shrugs and moves on.
What this thought experiment does reveal is that the use of Malleable Spirit and Release from Combat is something that's likely to happen after combat. The issue returns that it takes six hours to switch out Gifts so we'd need to be confident that we'd have chance to switch back. On the other hand, we can just keep captured ghosts and spirits on ice until we have time.
* Or, in extremis, we could even try this on the world soul itself, although we'd probably want to perform Sorcery to boost Thalassa's Gift.
Edit: Hell, I'm getting Spirit Ward, on the assumption that it works on ghosts, as they are spirits of the Dark Umbra for the Werewolf ruleset. A reduction of every spirit within a hundred feet's dice pools for every success rolled is amazing, particularly as the exclusion of totem and caern spirits should translate to excluding machine spirits. That doesn't just help Thalassa, it helps the entire group.