Hi redrock
in here https://pinside.com/pinball/forum/topic/williams-alternator-unit-optional#post-5488310 I wrote about what You can get in an non-manipulated Stardust.
The Coin-Unit should be called "How-many-players-want-to-participate-in-this-Game-Unit" and the Coin-Relay should be called "Start-Relay" - really just starting the resetting of units (primarily Score-Drums) - and in the end kicking out the ball into the shooter-lane ---
when "starting" for a second (third, fourth) player to participate: The Start-Relay does activate - but not triggering the reset-stuff - but stepping on the Coin-Unit (the How-many-Players...Unit).

Lets not talk about the 5-Cent-Relay --- the 25Cent- and the 10Cent-Relay come into action when the focus is "stepping-up the Replay-Counter".
25Cent-Relay speciality - when adjusted to "6 games per coin": Stepping up five steps on the Replay-Counter PLUS triggering the Coin-Relay (Start-Relay, see above).
10Cent-Relay speciality - when adjusted to "1 game per coin" - the adjustment is so that the 10Cent-Relay will not activate - instead the Coin-Relay (Start-Relay, see above) is triggered. Greetings Rolf

Hi redrock
great - Your pin is doing what it is built for. The following is rather "maybe of interest information": Lately in an German Forum the OP was complaining about his "Bally 2-Player Zip-A-Doo" - having a "two-coins-per-game coin-slot" but not working properly because the "Bally 4-Player Wiggler" he also has does behave properly. It turned out to be not a problem / fault --- Bally has built the Wiggler and the Zip-A-Doo different.
The behaviour was / is
Wiggler: Throwing-in a coin and another coin does directly start a (One-Player) Game. Throwing-in again two coins does start for a second player, throwing-in again two coins does start for the third player, throwing-in again two coins does start for the fourth player, throwing-in again two coins does step-up the Replay-Counter one step !, throwing-in again two coins does step-up the Replay-Counter one step etc. .
Zip-A-Doo (is a TWO-Player-Pin): Throwing-in a coin and another coin does directly start a (One-Player) Game. Throwing-in again two coins does start for a second player, throwing-in again two coins does NOTHING - these coins are just LOST (thrown-in - getting nothing for). Greetings Rolf