Regarding pieces which simply capture by relative positions, one Advancer
(a Pushme-Pullyu is too strong, and a FIDE-Queen not very different), one
Long-Leaper and one finely-tuned Archer seem enough.
The Swapper, which like the Immobilizer, the Chameleon or the Shield, is a
piece of its own nature, also combines two mechanisms, mutual capture and
mutual displacement, neither of which is very powerful.
Why not have the Withdrawer, which is worth less than half any other
officer, recruit an enemy piece it withdraws from, instead of merely
capturing it, provided that enemy piece has been previously lost?
(variant: it could convert by withdrawing one square, even if that piece
hasn't been captured for its side, and capture if it withdraws two or more
squares)
This raises the problem of Pawn recruitment. Should the Pawns be dealt
with individually, mirror-like, or could a Pawn replace any other Pawn?
I would suggest to make Pawns simply faithful.
(Unmakers require a similar decision.)