Moments after posting this, it managed to boot up successfully (eventually) in safe mode. So I tried it normally and it worked.
I'm not sure what solved the issue though. Possibly just fiddling with the innards and making sure everything is tightly secure I guess. I've had no trouble since... but would you recommend a reformat in case it happens again? It's due a big clean up anyway in all fairness.
Holding option brought up the menu for booting, but it kept trying to start up normally even when I clicked the disk.
I '''really''', '''really''' appreciate the help and advice. Thanks a lot. :)