Do they accept Us $$
Yes all the major supermarkets accept US $ at the current bank rate, but your change will be in pesos.
Yes. Not sure of exchange rate but should be posted, note change will be in Pesos. Exchange rate will be better with a credit card.
Yes and also you can’t give a huge amount of USD and expect to get a lot of pesos in return. They have a limit on how many pesos to give back to you per dollar amount you give them on your purchase.
Last winter at Walmart, they wouldn't take US $100.00 bills. 50's and 20's no problem. That may have changed.
"Exchange rate will be better with a credit card."
Right, assuming your card is charged IN PESOS, which is the normal way. The bank does the conversion at a decent rate.
Having your card charged in US$ is a sucker trap that's occasionally offered. Avoid that. (This is general advice, not specific to Walmart. Likewise, never withdraw US$ from a Mexico ATM, another trap.)
Also, it's best for U.S. Dollars to be in just about perfect condition. No rips, tears, bent corners or writing on the bills. I've had bills rejected at WalMart and Mega stores in and around PV for pretty tiny imperfections.