You sent me a message but I can't send a reply back so here's my answer to your questions.
I'm no expert on Linux nor rolling distros but from what I aware of, most Linux distros that use Debian as a base use sources.list
but ever since Mint 15, Mint uses sources.list.d/official-packages-repositories.list
So, no I would not recommended deleting anything. If it's not broke don't try to fix it. However, removing it should affect your system since your system reads official-packages-repositories.list
as your sources not sources.list
As for the bak file,I can't answer your question about that. I run Mint 13 and I don't have a bak file within my system. From what I understand about rolling distros, I would surmise that one would be automatically generated since the system changes so frequently compared to non-rolling distros.But, I could be wrong. You can ask about that within the Mint Debian section of the forum.viewforum.php?f=141