Other than that, I can't see why it would be unsafe, not to mention a way to ensure you don't cheat the weight up. Will make it easier to keep the bar under control. As far as delt fatigue, very little can be done other than to just accept it. To keep the bar in the proper position, you have to 'rack-it' on the front deltoids. They will fatigue.
Donkey raises work the same way as standing raises since the knee is straight.
The reason for the different muscle feeling the stress is related to the angle of the knee and it's relationship to the origin and insertion of the muscle. To test this do a calf stretch. The straighter the knee joint the higher you feel the stretch. As you bend your knee you feel the stretch lower in the calf and down into the achilles tendon. Sitting down in a chair with your feet flat on the floor raise your heel and flex your calf.