How to setup and process loan details and payments for S-corp stock sale via seller financing with payments being deducted from pass-through distributions

The sale of equity is an outside transaction despite there being nothing wrong in principal with the new owners paying the previous owner from their net profits (presuming these continue in a fashion conducive to meeting the obligations)

If you want owner A equity to transfer immediately in full I suggest keeping the loan and sale outside of QB. Enter the transfer of equity (current stock value as of date of sale) from A equally to C and D, essentially making each of them 25% owners (if A and B were equal partners). When it comes time to make payments to A, you do so with company checks but deduct the payment amount from C and D equity equally, remembering that retained earnings not distributed adds to stock basis and then that basis is reduced by distributions (you can run a separate distribution account similar to member draw and balance everything annually. Keep track of loan details including principal and interest outside of  QB sicne it is not company money but shareholder money making the payments.

One way to create a single check is create a vendor bill for A, posting two items that in turn each post from C and D retained earnings (you could set up sub-accounts of retained earnings and thus track actual profit distributions by shareholder)