Just tried with my wife's POSB debit card with her wechat account linked to SG mobile number, no difference, still can enable the wallet but not receive ang bao, again the 二维码收款 trick works fine.
So I think with the experiences so far the above tricks work and it is good enough for small transactions during short trip to China.
1) Just need to find a friend with a working wechat China wallet to send you RMB1 ang bao and then add your SG credit/debit card to trigger the wallet function. You may still not able to receive the RMB1 ang bao and it will go back to your friend.
2) Find the same friend or others to top up for you by using the "Receive Money" or "二维码收款" function.
Chinese menu option:
我 -支付-收付款 -二维码收款
English menu option:
Me - WeChat Pay - Money - Receive Money
3) After that you can use the balance 微信零钱 to send ang bao or pay street vendors by scanning their QR code using WeChat.