I have a 1910 house and am converting a pantry to a 1/2 bath (toilet and sink only). The walls are 12 feet tall and are sitting on 8" X 8" timbers as foundation. Some walls have diagonal studs along with the vertical and make it's impossible for me to direct a 2" pipe through them, up to the attic and through the roof. I -can't -directly vent straight up from the fixtures but I can do down into the crawl space and then if I there is a wall without the diagonal obstruction run it back up and through to the roof. Toilet and sink will connect to 3 inch drain to septic tank.
1. How much can I stretch the critical distance from toilet to vent. I have read that it's 6'

2. Since I can't vent in the wall directly behind the sink - are there any problems that anyone has had with running the pipe in the crawl space to connect to the vent from the toilet?

Thank you.. any help is appreciated.