I have seen folk lying on their back with their foot on the peg, knee bent in the air, to try and shoot an elevated shot; it seemed a fair interpretation of a prone position to me.
I too thought the high 20mm shot was a good target, testing physically without making you lie in a 'contorted' fashion (sideways is comfortable as has been said). Especially it made you think hard about where to aim.
For the darkened target, imho you could see with the naked eye where the kill would be, in the middle of a symmetrical object, so it was really a test of judging the distance.
It's the Indian not the Arrow