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I am not sure whether [allow[ing] moves of one piece to be relayed from Friend to Friend through moves borrowed from another piece] was the intention of the original definition

It seems none of the info on the CVP, including Orphan resources (which could in principle provide analogous situations) such as the Helpmate problem and the (Wayback Machine copy of) the Torsten Linss problems (linked from the Piececlopedia article), is able to clear this up, and short of finding back‐issues of the British Chess Magazine info on the web seems very elusive

The only other source I could find offhand was Die Schwalbe's glossary, which also doesn't really clear anything up (though it does give rules for initial pawn steps, at least for Orphans: an Orphan may make a double step from its second rank, but only a single step from its first)

Among the freely available Schwalbe issues, a few feature Orphans but apparently none feature both multiple normal pieces that could relay distinct moves and Orphans on both sides that could chain. Friends seem to be completely absent from these issues (indeed, any of the issues whose indices are available), though still present in the Glossary (which clarifies that, at least for the Schwalbe, the Friend explicitly can't promote with a pawn move to the far rank)

Would be interesting to see whether the BCM or Brown's own problem collection made any use of this that could clarify this, but access to either is probably a pain :‌/

It would probably be better to use a 'static' definition, like "when it would be able to capture the Friend if that had been an enemy pawn".

If only direct move pass‐through is allowed as you suggest, this seems uncontroversial; in the case with indirect relay I'm not sure whether that makes the non‐reciprocal passing on (assuming it remins desired — I quite like it myself) more complex to formulate correctly than reciprocal passing on. In principle that same question arises for the Orphan too, though I imagine the chances of finding a problem that relies on either of those possible behaviours is incredibly slim

[The Ghost] captures by "passing through" an adjacent square on its way to the next square beyond

From the diagram I take it this is allowed only when moving to Alibaba destinations?