Electronic – RS485 ground noise problem

I'm guessing that the pulsed current draw on the +12v supply is what is putting the noise into the RS-485 signals. For debugging purposes, try disconnecting the LEDs. If your noise issues go away then you know that it's that pulsed 1 amp current. Assuming that's the case, then your job will be to reduce the current spikes on the +12v wire (which are also on the GND wire). A filter using an inductor+large_cap would help. Using a different cable where the power is isolated from the data could help too.

You're running in half duplex mode, joining Y to A, Z to B right? Why do you mention A- and B+ and A- and A+? I'll assume you just mean A and B, and that the termination resistor is across them.

Each receiver at the end of the 485 wires require 120ohm termination resistors, so in half duplex mode you need one on either end since both are receivers. Do you have one on the FTDI end too?

485 transmits 1/0s by separating the pairs by a voltage, why do you want 'failsafe' resistors that will tie up the bus? In an idle state the voltages should be the same between the two, which is what the 120ohm resistor also helps to do, you're working against this by pulling the separate lines apart and wasting power. You have 360+360+120ohm from VOUT to GND permanently.

What should the polarity of the stop bit be? Check the TX pin polarity of the UART and make sure its idle is 0 during the delay period(s). If it's transmitting a 1 it could be erroring because the FTDI receiver is never seeing the stop bit?(see update below)

The voltage levels themselves don't matter so much as there is a >200mV difference between A/B. The FTDI chip should have no problems determining what you're sending with a 3v difference, just as your receiver did with a ~2v difference.

Update: Since realizing that you are able to actually receive correctly the first few bytes before the bit error, this looks more like a timing issue. Try run at a lower baud rate and see if you get any more received data before the error.