>> In the "Website Payment Preferences" settings, if I turn on "Auto Return", would it screw up my non-dap Paypal Payment buttons that also use the same Paypal account ? <<

If you use dap generated buttons for paypal standard, then DAP requires that auto-return be globally enabled in "Website Payment Preferences" because without it, dap cannot auto-login users upon purchase.

Turning on auto-return would NOT impact any existing payment buttons that set it's own button-level return level.

If you use dap buttons, then dap automatically sets button level return. So it's never a problem if you use 1 paypal account for multiple dap accounts. You can set autoreturn to point to one of the dap sites but use that same paypal account to sell products from multiple dap powered membership sites. Does not matter what the auto-return is set to.. it just needs to be set.

Say you use non-dap buttons, and a 'return' is not set at button level, and the users that buy using those buttons currently end up on the paypal page after payment.
But if you set auto-return in paypal, then if a button-level return is not set, paypal will end up returning users to whatever you set as auto-return url. So the buttons that dont have button-level return url, will be impacted.

To summarize, setting auto-return should not impact any buttons that has button level 'return' set. To be sure of this, after you enable auto-return, pls run a test using your existing buttons and make sure it works as before.

If your buttons get affected by autoreturn, you can use 'paypal-hosted' buttons and integrate that with dap instead of using dap generated buttons. This way you do not have to set auto-return.