Hay,
I have seen a promo video from dji which showd that a p2v+ v3 can carry 4 phantom shells. Based on that observation, I believe 300 grams will not be a problem.
Btw Your girlfriend must be great getting a ring with that big (3oo gr) a diamant! [emoji6]
Recallibratingil of the compas is only required when flying in a place exceding appoximately more than 30 miles away from the place you previously callibrated the compas. Doing it more often does not serve any purpose and therefore only introduces only more chances something goes wrong.
Att indead gives you a possibility for manual control. The advantage over manual mode though is that the altitude sensor is still operating, which make it far easier to fly compared to manual mode.