How about an electric plumber's snake? These electric augers can be rented from any rental store, and are usually at least 100 feet long. You can have someone above operating the motor as you feed the auger into the drain. I would again reverse the water flow to blow out any debris that may break up. Not very familiar with the construction of the main drain, so this is a shot in the dark! Good luck!
P.S. If you use anything electric be sure to plug it into a GFCI protected outlet and make sure the operator is familiar with the tool....those snake can easily get away from you!!