Ok... I gave up and ordered the UAV strong arm.
I have been aware of cracks since the P2 range and loosen and re-torqued my screws from day 1. Spring is here and I cant afford the 2 month wait so I took matters into my own hands.
My phantom has had 3hrs of flights, no crashes and always hand caught. Majority of the flight was in cooler winter climates. The cracks I have are motor mount related. The 2 shell mounts are still good and has no signs of cracking.
Being a hobbyist, I did some tinkering and preventative measures to minimise the cracks. I used cable ties on the end of the arms to make it more rigid and I used hot glue gun to reinforce the plastic - worked on the P2 but doesn't work on the P3. I did a few more flights and more cracks started to show. Some cracks are invisible and require closer inspection. I found mine by licking my thumb and rubbing the plastic. If it shows up black, it means you need to inspect and its most likely a crack. If the crack joins from screw hole to screw hole, then there is a high chance of catastrophic failure.
It takes 1-2 months for the UAV bits to arrive. For now, I made some temporary brackets to stop the motors from total separation. The brackets are mounted on the vertical screws as these have the same camber and depth of thread.
After messing around, its come to my conclusion that the shells are using a poor plastic compound. It is possible that DJI has used other parties/contractors to manufacture the shells and never quality controlled.
The only thing I will hate is that DJI will be making a **** load when they release a revised non-cracked replacement shell. There will be end-users like myself who can't afford to lose 2 months of seasonal downtime and have to resort to voiding the warranty and paying full fare for a new shell because of a fundamental design fault.