Well I just disconnected the whole motor/vape assembly and now I get a power LED on the Arduino working at least but all the components are still on full brightness... reading 2.8V across VIN and GND
With 9V wired to Vin, per your schematic, you measure 2.8V from Vin to GND? I must be misreading this.
yes VIN to GND Pins on the Arduino.
Then disconnect everything but the Uno, and download the blink sketch and verify that your Arduino itself isn't fried.
@jim-p I'll stay quiet a minute, I see you've been trying to contribute.
Done. Still works fine.
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The other way around but the big problem is that you applied 9V to a digital I/O. That will almost cetainly damage the arduino even for a small current.
Even if so, it worked when USB was connected... LEDs should be dimmer but at least work right?
There is no guaranteed Rds(on) specified for 5V for the 740 mosfet. The ones you have now may partially work, the ones you buy tomorrow may not.
why not btw? At max. power the strip will draw ~13W/meter.
Each color can draw about 18mA max. So for each pixel, thats 3x18=54mA. Then, 54mA x 120 = 6.48A Your 6AA won't last long.
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D8 may still be toast, though the Arduino works. Can you jury-rig a resistor-LED combo to D8 with the Arduino out-of-circuit, modify the blink sketch, and verify that the output works?
I'm still pondering a 9V to 2.3V drop in your circuit wiring, by the way. I don't believe it, because that implies 6.7V drop across what should be a wire, which in turn implies WATTS of heat. Usually, this sort of thing implies to us that the user doesn't know what, or where, he's measuring the parameter, so I have my doubts. Sorry, but it's the truth.
Ok.. I will only use one color (green) so 18 x 120 = 2,16A and only at short times...
It has to be super portable so the other alternative would be a powerbank (but they only have 5v and 22Wh max.) or something like a rc battery?
The whole thing has to run like 200 seconds at a time so it might still work with AA batteries?
It still works.
Well let's see. A good AA battery may have a capacity rating of 2.5Ah. So each battery will be able to supply 1.25V x 2.5Ah = 3.13Whrs of energy. 6 will yield 6 x 3.13 = 18.78 Whrs of energy
Your LED strips will use 5V x 2.16A = 10.8W of power.
So, 18.78Whrs/10.8W = 1.74 hours or 6264 seconds
6264/200 = 31.
So if you operate it for 200 seconds at a time, you can do it about 31 times
NOTE: I used 1.25V for the AA battery and not 1.5V because 1.25 is the "nominal" voltage during the battery discharge.
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That's perfectly acceptable for me 
When you measure 2.3V at Vin, what do you measure at the 5V pin of the Nano?
Depending on where I measure I get different readings.. shouldn't GND Pin and Circuit GND be the same??
Something is seriously wrong with your circuit. Is this a PCB, a breadboard, loose wires, or what? A photo, please, would be useful.
Here is a picture of the original circuit with the motor/coil connected (bottom cable). The Laser and 3 small LEDs are connected to the upper Mosfet, a LED chain is connected to the lower one. The LED Strip is connected to the left side cable. Let me know what parts you need to see close up..
Comments on what I see:
- You may be running multiple amperes through breadboard connections; avoid that like the plague
- Our discussion is about the power connections to the Arduino, yet I can't see where the green wire connects in the photo
- Any time you find yourself wiring a multi-watt resistor into a breadboard, stop and think about the currents involved, because it's likely you're overcurrenting the breadboard connections
I would focus on figuring out why you have a high-resistance connection between the GND pin of the Arduino, and the GND of the power supply. You're right, your measurements are either done incorrectly, or are telling you you have a circuit problem. However, frankly, if you fix that circuit problem given the other issues noted, your immediate result may actually be to fry something.
Good luck, I'm offline shortly and won't be back for 12 hours or more.
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My calculations were just for the WS2812s, your motor and vapecoil may drain the batteres very quickly.
It connects to GND rail right beside it. It may seem more like black on the side of the photo...
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The motor/coil will only be active for a few seconds...
OK then.
Your circuit still need to be revised