I have a slurry mixing package which has been installed such that the EDUCTOR discharge  has to overcome a back pressure head of 25 ft before entering the tank destination.
The EDUCTOR has a 2" nozzle and the supply is from a 6" feed line with a flow rate from a pump of 1000gpm, capability of 100ft head. The discharge from the Eductor goes to an 8" manifold but has a T-junction and several 90 degree elbows.
 Manufacturer recommendation is for a supply of 50 psi to the Eductor and minimum back pressure for efficient operation.
 I have to mix a slurry with a weight of up to 16 lb/gal (sg approx 2.0)via powder into a hopper of the Eductor.
 My question is , will the Mixer work effectively against this back pressure .
 I already have had problems with back flow into the Hopper when cutting off the supply because of the pipe and tank elevation. Fitting a check valve in the discharge will only add to my back pressure problems.
 Normal practice  is to fit the Eductor package on top of the tank to prevent back pressure. But I cannot change the location of this unit.
 Appreciate any feedback from people who have come across this problem before.

So far as I know, the first eductors to enter commercial production were used to feed water into a steam locomotive's boiler.  The boiler's steam output was the motive force.  The Sg was around 1.0 of course, but the head was way more than 25 feet.  You've already determined that you have an excess of motive flow available, so the eductor should work okay while feeding powder.

The problem you're trying to solve has nothing to do with dynamic head loss in the discharge pipe.  It has to do with the discharge pipe being full of motive fluid when you shut off the pump, and not having anyplace to go, except into the powder hopper.

So, give it a place to go.  Put a dump valve in the eductor discharge line.  Open it before you shut off the motive fluid supply.  If it's big enough, the discharge line will drain through it instead of filling the powder hopper.  

Does there happen to be such a valve already in the manifold you mentioned?

Have had similar experience with mixing milk powder and returning to a silo, much higher static head. For that application I used a booster pump post the eductor to ensure the throat pressure did not become positive and end up with wet powder in the mixer. This is not your problem, just for interest sake. The other feature that will solve your problem is installation of a valve on the powder feed above the eductor and below the hopper. Thsi valve is closed before the system is stopped. I used a buterfly valve for cleaning reasons, a ball valve would be appropriate for your application.