But I saw at least two that were far longer and maybe brighter that followed a different trajectory, one streaked toward Cassiopeia. Perhaps these were random events not related to the Perseids?

The radiant is in the north-west part of Perseus, between Cassiopeia and the horizon from the Northern Hemisphere this time of year. So anything that seems to come from that area is a Perseid. We also saw a few coming from the "wrong way" and one of the guys in the club documented them and sent the report to the meteor tracking site.

Staring at the sky at night is a reliable way to see something incomprehensible and wondrous. Can these flashes of light really be the result of metal flecks the size of sand grains? Why are they usually nickel and iron? Are they really roughly parallel, like Crepuscular rays, despite apparently diverging?

The ones that leave a trail are about a single millimeter is diameter. Most visible meteors are microscopic in size. Remember these things are hitting the top of the atmosphere at about 50-70 KM per hour; at that speed the air cannot get out of the way fast enough so you get a bubble of compressed air in front of the particle. This air gets compressed, which heats it. That air gets so hot that it vaporizes the surface of the object entering the air. For something very small, smaller than a pebble, the object literally vaporizes and becomes dust in the air somewhere about 30-50 miles above sea level.

Are they really roughly parallel, like Crepuscular rays, despite apparently diverging?

Yeap. The dust grains are impacting the atmosphere at a near 90° angle to the tangent of the top of the atmosphere. There are exceptions, but not many. Chelyabinsk entered at a shallow angle which is one reason it exploded in the air rather than on the ground.