How to sprout quinoa (and other grains) safely
As you stated above that you have used every possible method, i use simple method for grain sprouting. First i wash grain properly to remove dust and then soak them in normal temperature water for at least time mentioned in recipe.The first step in preparing quinoa is to remove the saponins, a process that requires soaking the grain in water for a few hours, then changing the water and re-soaking, or rinsing it in ample. So, continuously changing water after ever 3 hrs to keep them fresh. Then followed by your usual sprouting method. But for sure there must be the problem with the quality of grain batch which may be due to fertilizing,soil or any other reason. You better purchase from some good quality grains then follow the usual procedure of sprouting. 
It happens sometimes, i had been through same once but till that stock lasts in my kitchen. There after i went to have good quality grains and issue resolved. Hope it would help you out.