If you can see stuff coming out the return jets my guess is there is something wrong with the filter. On the other questions, I usually wait 20 minutes or so after adding DE before I vacuum. It takes some of that time for the pressure to stabilize. I also run with DE all the time, usually. After backwashing I chuck in more DE. Something you can try, when you add DE see if the white stuff comes out the returns. Try a cup or so...won't hurt anything. If it does then the filter has a problem. Could be a broken lateral at the bottom of the sand bed. I see in your profile you're in North Texas. Don't know the Texas Winters but could you have freeze damage? I doubt it. Also, when vacuuming settled silt I've never seen it come out the returns, even with just plain sand. It does get through but I can't see it. Best I can do for now. Keep us posted.
Al