Is the Filter Theory committing the ad hoc fallacy and is it unfalsifiable?

Yes, certainly, correlation is not causation. However, the actual mechanism of the memory impairment can be interpreted via the filter/transceiver theories as indeed due to the brain damage, but where this is because the spirit is very intricately intertwined with the neuronal microstructure of the brain which it utilizes to manifest in the physical. Consciousness and memory are therefore functionally impaired by the physical neuronal brain damage, but in a model where the mind is not equal to the brain. When the spirit finally disentangles itself from the brain structure, mind, consciousness and memory are no longer impaired. Regarding memory, this explanation of course also requires that human memory data not be exclusively stored in the brain, but either solely in the spirit/spiritual realm, or in both places.

I might add that I think this conceptual approach neatly accounts for a number of the mysteries associated with the mind-brain problem. Such as terminal lucidity and paradoxical lucidity, where there is an unexpected return of mental clarity and memory, or suddenly regained consciousness, that occurs in the time shortly before death in patients with severe psychiatric or neurological disorders (terminal lucidity), and where this occurs long before death in cases of paradoxical lucidity. There are especially mysterious instances where this sudden return of mental clarity and function is despite severe brain damage.

This hypothesis is based on the filter/transceiver theory and would partially agree with the materialists: the mind does in fact depend on the physical brain as much human experience seems to indicate - but in this hypothesis that dependence is temporary, only during physical life when the spirit is inhabiting the body and manifesting in the physical mainly via complexly interpenetrating the brain's neuronal structures. During episodes of terminal (and probably paradoxical) lucidity the spirit has become partly decoupled from the damaged physical brain and is able to temporarily experience a more normal consciousness including access to memories, but still has enough connections to other brain functions to communicate vocally.

Cases of near normal mental functioning despite loss of most of the cerebral cortex due to severe hydrocephalus could also be explained by this hypothesis - it is functionally similar to terminal lucidity in that the mind appears to be fully functioning despite loss of or damage to much of the brain.

This hypothesis accepts that there is a dependence of the mind on the physical brain, but postulates that this dependence is strictly only during physical life.