Your rpm/mph sounds about right. It may fluctuate a bit, then stabilize when the torque converter locks.

Your voltage reading is also correct. The electrical system is nominally 12 volts, which is the voltage of the battery. In order to keep that battery charged, the alternator must put out a slightly higher voltage---I believe the normal range is between 13.2 and 14.4 V.

Don't worry about the white plastic clip on the speedometer connector. The connector itself will stay on just fine without it. The dealer is correct---its only purpose is to guard against tampering, which it doesn't do very well, as it is possible, if you are careful, to remove it intact and reinstall it. Older cars used a twisted piece of wire with a lead seal on it, which is much harder to defeat. You may find one of these on the speed sensor connector on the rear axle.