How can I resolve drain issues in a home with no venting?
If you were to remove a p-trap from under a sink upstairs, you'd basically be opening a vent. If you do this I doubt you'd notice any change however. You sound like you have a blockage in your drain line. Lack of a vent isn't going to force your entire drain to hold water like that, it would just prevent a full volume of drain flow from occuring. You need to snake your mainline, the line leading into your septic has a blockage somewhere. That is your actual issue.