In my opinion the spokes of the "new" MP5 are too thin... I wish.. I could buy thicker 3mm spokes.
This is where our opinions vary, as in my opinion, the spokes of the "new" MP5 are still too thick for conventional bike use.
I would much rather have standard 14g 2mm thick spokes on my wheels, but the holes in the flanges are too large to allow them to be used, so the only valid alternative is to used butted spokes:
Philip, what diameter spokes were fitted to the original rear wheel
(and front wheels), and did/do those regularly break too?
Thinner spokes will often enable a greater amount of road shock forces
(bumps and pot holes etc.) to be absorbed by the "more elastic" tensile properties of the thinner spokes.
If the spokes are too thick, the stretch available within the spoke is greatly reduced, and this can produce a very hard and unforgiving ride on a "hardtail" bike or trike.
Today I received 37 new spokes, but they don't last too long. I brake them. Also often nipples fall into the rim. Because the aluminum rim flexes too much. Golden Motor could make a steel rim optional. For tricycles... 
Of course I know it's because my MP5 wheel always stands straight on the road and gets a lot of force when road is not 100% flat and of course in corners and road bends.
I fully appreciate that your particular trike places a lot more load on the wheels than most conventional bikes:
But my bike is extremely heavy... It weighs with me..... empty 330 KG. The cargo can weigh as much as 350 kilogram!
Although 680kg is a tremendous amount of weight, the majority of this weight will typically be placed directly over the front axle
(and front wheels), not the rear wheel. I would expect
less weight to be acting upon your rear wheel than is usually acting upon the rear wheel of most heavy duty adult tandems.
If you place a set of bathroom scales under the rear wheel and then sit on the trike, you will be able to see exactly how much load the rear wheel has to withstand, and I would expect the standard Magic Pie spokes to be able to cope with this load
if they are correctly fitted and sufficiently tensioned.
If your spokes are breaking at the elbow:
Cracks like the ones visible in the above photo are typically caused by metal fatigue, as the elbows will constantly flex with each revolution of the wheel, basically caused by a lack of support between the inner elbow radius and the rounded/countersunk edge of the flange holes:
I strongly recommend that you consider refitting the spokes on the outside of the hub flanges so that all their heads are on the inside as shown here:
This enables the spoke "elbows" to be better supported against the flanges, thereby reducing the likelihood of spoke failure due to fatigue, and it will also allow the wheel to be able to withstand higher lateral forces, which are typically much greater on a non-leaning trikes like yours.
Spoke nipples should not undo themselves and start rolling around inside the rim if the spokes are initially tightened sufficiently, and are also correctly re-tightened as necessary as the new spokes settle in, inevitably reducing their initial tension.
Alan 
