Jenny is using an Olympus SLR camera when doing Alan's portrait session. The shutter sound is not that of an Olympus which has a distinctive "ker-klug" sound. Though is is still a focal plane shutter and not a leaf shutter, it most assuredly is not the recorded sound of an Olympus focal plane shutter.
When David discovers the marijuana and ecstasy in Alan's bag, he picks up the marijuana in his right hand, and the ecstasy in his left. We cut away to Alan for a couple of seconds and then the items have switched places when it returns to David. Then they switch back and forth again, before David throws them over to Alan.
When Alan burns the barn, you can clearly see the donut shaped bruise on David's forehead, however in the later scenes after the burning of the barn, while David is in the car, and later in the mansion the bruise varies between being gone, or being very faint.