***[SOLVED]***
Hi everyone, I wanted to say that this has now been solved! So after another week or so contemplating whether or not I should buy a new motherboard, I took the risk, I got one off ebay for my a1466, it was about 70 dollars so not cheap, but I knew that I had to try something before throwing the MacBook away (and because I didn't want to give up on it). So, I put everything into the laptop, I tried starting it up (by holding power for 10 secs, then pressing the power button normally) and worryingly when no battery was attached nothing happened, but I knew I was on the right track because differently this time the screen flickered a tiny bit and I could see it was lit up in the LED, like a dark blue/light black, which was a good sign.
Trying this again a few times I was losing hope but I'd realised I hadn't put the left speaker periphery cable back in, so I did that, and because it was my final shot I thought I'd plug the battery back in and charge it a little. So I did, and after holding the power down for another 10 seconds or so it started up!!! ***The Frankenstein MacBook is alive!!***
I got an error when it turned on, and it had to restart because it said 'your computer has restarted because of a problem', I think this is fine though because all of my files are exactly as I had them months ago.
Hopefully, this thread will be useful to anyone in the future looking into why their MacBook isn't starting. Lots of late nights, and hours spent on it. This marks the end of the around 6-month struggle of this MacBook not turning on properly, I'm so relieved to have it back :')