@Sunseeker According to the Connecting the Radio page, D2 is the default pin for the CE signal for the NRF24. If you'd like to use this pin for I2C, you can free it up by assigning a different pin for the CE signal by adding this line to your sketch:
#define MY_RF24_CE_PIN pin
IIRC, all available pins should work for this purpose (D0, D3, D4). Just pick one and if it doesn't work, use one of the other pins. Please make sure to add that line before #include <MySensors.h>.
The choice of MOSFET can be tricky. Seems that irlz44n was discontinued so you may need to find one adapted to the voltage and current you want to handle.
In all the cases you need to get a logic MOSFET too, meaning that they are fully open usually around 1 to 3V.
Also I'm sort of confused as it seems to me that L7812CV is a Voltage regulator, not a MOSFET. So if you are actually using that in the MOSFET spot it will definitively not work.
Especially if you are handling high current or voltage I would recommend using one from a reputable source for your MOSFET, I recently switched from mouser / digikey / aliexpress to mostly use http://www.arrow.com as you get free regular shipping, event if you order a couple of components (no affiliation to them whatsoever).
