My house was designed and built around 1920, so I doubt I have any modern design of the system. But all of my drains work well and we don't get sewer backups, at least in the five years I've been there. The vent stack is about 10 feet away near the washing machine.
To test the viability of the cleanout, I sprayed a hose into it full strength for about 20 minutes. The water just drained and drained and drained -- no sign of hesitation. The way I'm envisioning, I think the P-trap will keep the sewer gases contained like it should. I'm pretty sure it will all work out. At any rate, I'm not finishing the basement until spring, so I'll have a few months to make sure it doesn't get wet or gassy down there.