We need to be careful here.
The One Pay Debit card is issued by Coastal Bank, and basically is a reloadable debit card (i.e. not backed by any of the possessor's bank accounts, therefore similar to a BB card). You earn cash back for appropriate purchases at WM...i.e. no GCs or prescription drugs,,,I think they do allow OTC meds) and is limited to 3% back (max $150 spend a month). After that, the program is bupkis....0.1% back

The WM One Pay CC is completely different. It is a store based MC credit card (just like a Kohls or Macys CC) issued by Synchrony Bank. You apply for it just like any other CC with the usual credit pulls to determine credit worthiness. That means there will be a credit limit, and I feel reasonably confident that, given this is WM and WM customers, that limit will be really low.
There is no loading of this card. It is basically a rewards CC with 3% Cash Back at WM (you can read the restrictions on page 14 here). If you have a WM+ account, you earn 5% back at WM only).

IF you are turned down for this fabulous card, which I suspect many WM customers appying will be, then Synchrony has a back up card...the WM Spend card. (Below from the website)

The OnePay Walmart Spend Card is a credit card that can only be used at Walmart and Walmart.com. When you apply for a OnePay Card, you will first be considered for the OnePay CashRewards Card; if you do not qualify for a OnePay CashRewards Card, you will be considered for a OnePay Walmart Spend Card. You may not apply for the Walmart Spend Card directly.

The Walmart Spend Card does not have a standard rewards structure, but OnePay provides Walmart Spend Card customers with limited time offers to earn OnePay Points.

So, to your interesting question
The reason for shut downs in Coastal One is the appearance of MLing activity. Will loading Coastal One and paying off Synchrony One with it be possible...yeah, but I have not used my Coastal One account for any BP. I shop at WM and spend less than $150/month there, so between cash withdrawals each time I shop and actual purchases, I am easily able to unload the card and it appears to WM and Coastal that I am a great customer. I also get 3% back at WM. Ce n'est pas mal.

There are plenty of DPs out there where paying bills, especially CC bills, raises the hackles of Coastal One. But even if that weren't the case, and presuming you qualify for the Synchrony One Cash account, you will be limited by your credit line,,,,and I honestly suspect that will be less than 4 digits in amount to start.

IMO, my Coastal One account isn't worth the 3% return risk (and remember Coastal has a very nice savings account program to boot) but to each their own. Let us all know how you fare in this adventure.