Hi Andy, welcome! Looks like you sorted out b)... as for a):
You can purchase credits and play for credits, but they are not redeemable for cash (even though a player's profile indicates earnings in $ (not sure if other countries indicate their specific currency). Credits can only be used in the game for pro shop purchases, tourney/match entry fees, gifting to other players etc.
For the indicated dollar earnings, I just multiply by 100 to see how many credits I've earned for tourneys, milestones, matches etc., which is different from the amount of credits you actually have due to purchasing/spending credits in addition to the awards.
To use other ways to purchase credits, click on get credits/buy wgt credits. Then scroll down and click on "change billing information" to get the various choices, including PayPal. -Keith