The ADXL345 should definitely be able to run at 400khz. If there really is a problem running it at 400khz on the Edison I would hate to just patch around it and run it at 100khz.
FWIW I tried a MPU9150 breakout board last night with the same results. I tried connecting it to both the A4/A5 and SDA/SCL headers with the same results. I also tried both with and without the commands to enable I2C6. I did see one kernel crash as you showed previously when detecting on I2C6, but mostly it just listed an empty table.