That is due to a petal blight. Fungicides applied immediately after rainy periods can help reduce that problem. If you are building an arbor about 7 feet high put the posts into the ground about 2 or 2 1/2 feet deep. If you use concrete to set them 2 feet is plenty. The arrangement sounds okay although I am not sure I am picturing the exact thing you have in mind. I would start the fencing about 3 feet from the ground.
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