You got it exactly correct. When you are doing things with codeshare partners the rules apply for the program you are earning them in, not the one you are flying on.

So if you book on CO.com and then earn the miles in WP you will get 50% for the low costs codes and 100% for the higher codes.

This applies to any CO OPERATED flight, even if you book it on NWA.com or directly from CO or NW. Any CO metal you fly in will only earn 50% in WP unless it is the high cost ticket.

NW makes the rules for WP and they are posted on NWA.com, CO makes the rules for OP and they are posted on CO.com. Often they are similar, but the one providing the miles always has the say.