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The originality of Conquer seems (to me) the fact that the "re-introduction" square is the square where the capturer went from.

You are quite right there and as you have already said, the rule is new. And such a clear rule is, in my opinion, positive for the strategic orientation of a game.

The variants 'Reinforcement Chess' and 'Chessgi' quoted by you from Pritchard's CECV differ here.

For 'Reinforcement Chess' it reads:

A captured man changes sides and is immediately replaced by the
immediately replaced by the player who
replaces on any empty square, with 
two restrictions: ...

For 'Chessgi' it is to be read:

Like reinforcement chess, but it is not necessary to
to reinstate a captured man immediately; instead, the captor may
instead, the captor can hold him in his hand and then
place him on any vacant square, instead of making a
making a normal move. 

In my opinion, the above variants are hardly predictable for the opposing player with regard to the reinstatement of the captured piece. My variant/rule is more pragmatic and more calculable.

By the way: The variants 'Reinforcement Chess' and 'Chessgi' were not known to me during the development of 'Conquer'. And I think that the rule of 'Conquer' is original and new compared to 'Reinforcement Chess' and 'Chessgi'.