I think that IS the easiest way to pay a mortgage, online and with the user's account. What I do though is change the user's email address and password to something that I would normally use. I will give the seller the new information, but they usually dont even want to know about the house anymore, just that its being paid.
For owner financing deals, I pay the seller directly. For unsophisticated sellers I write a check every month and mail it to them. For more advanced people, I set up an automatic draft out of my checking account. Sometimes people like to see the check and go to the bank to cash it. I think it makes them feel like they just got paid, like at their job(because that is what they are used to).
Its kinda the same with people paying me. Some will get a money order and take it to me or my wife personally, some do cash app, one deposits directly into my checking account and takes a pic of the receipt, and I have one auto draft. Now I have one that is with a servicer. It is what everyone is comfortable with.