Pretty sure that replacing the rear lights with LEDs have lowered the resistance (since the LEDs don't draw as much juice) so the flasher thinks there isn't a load on the rear lamps because your bulbs are blown. You might need an inline resistor for the rear turn signals to make it flash with normal frequency.
That is correct. however other users that have the same spyder led taillights have said they are built in. They were on my Spec D Tuning brand as well.
I would check to make sure there is a resistor box on the back of the housings.
Also make sure the fronts are done correctly as well.