[werk_ag]

First of all, judging by the photos you have done a really nice soldering job.

It's not easy causing damage to the IRL244N just by soldering it.
Maybe, there isn't any major problem. What kind of LED are you using? Some 3mm leds works with very low current and blow easily with 15mA, if you are using this kind of LED, increase R1 and R2 to 2.2K

You can do a small test with your LED's, with just a 9V battery. Connect them with the 510 Ohm resistor you are using, if they work well and don't blow, then they also will work on the TX board.

I discovered the TX led was not blinking

The TX led blinks (switches off and on).

This confuses me, there is any mistake here?

Could you please just check, what voltage you have in the output pin of the 7809 when the board is powered by 12V?

FAN being feed 4 and something volts while no 12v source is present

You are measuring this between the two fan pins, or between the +9V fan pin and GND? If the last case, the reading in normal.

7809 regulator having a reading of 4 something volts on the Output side while the board is being powered by USB only

This is normal, when the board is being powered by USB only. These voltage comes from the Vin pin on the Arduino.