White powder in pool, need advice for fixing.

[PoolDoc]

I'll post the pics, and reply more carefully later. But before I do, wash off the DE membranes, and inspect them carefully. With an old filter like that, it's to be expected that some have developed holes.

I've never seen that filter before, but I'll answer some of your questions as best I can:

+ Normal function on a DE filter is for DE powder to swirl around inside the tank, after you add DE *or* upon start up after the pump has been off. During this period, the DE will gradually 'plate' itself on the membranes and remain stuck there till the next pump-off interval. During this period, DE will exit the filter through any opening through which water flows EXCEPT the membranes and the air relief filter. Any openings through which water leaks in the piping, the relief line, or in tears in the membrane will result in DE in the pool.
+ part 10 (4, 10, 14) is the air relief line.
+ parts 5,6 seem to reference typical membranes or possibly underlying support grids
+ the membrane assembly seems to attach via friction fit. You might check for an internal O-ring, but I'm guessing there is not one. If it IS a friction fit, it's quite possible the PVC ell has worn enough to allow bypassing and DE leakage.

[PoolDoc]

PS: I'll wait for the results of your inspection before attempting to guess at other causes.