MacBook Unibody Model A1342 Battery
I absolutely severed this cable when loosening the adhesive around the volume button area. There is really no way of knowing if this cable somewhat elevated from the frame and therefore susceptible to being cut, and it happens to be right where one starts to remove the face with the guitar picks. The cable is directly in from the down volume button, so be extra careful not to put the guitar pick in too far in this area. I recommend buying a power/volume button cable just in case. Replacing this cable is also not easy, but doable. Just try to avoid doing it.
I also had difficulty installing a 2.5" SDD using a Sabrent 2.5" SSD & SATA hard drive to desktop 3.5" bay bracket. The cables inside the iMac are just long enough for the original 3.5" drive and not long enough to reach a 2.5" drive in this particular bracket. I had to settle with an SSD held into the bracket at an angle with two screws. Somewhat shameful, but it's not going anywhere anyway.
I had the same alignment problem, too. I ended up removing the rubber grommets from the plastic enclosure, widening the holes with a small file, then replacing the grommets. All four screws went in, but not in perfect alignment. Should be ok, though.
Why no mention of attaching the new hard drive to the Optical Drive Enclosure using the enclosed phillips screws?
Once you have securely inserted the new hard drive into the Optical Drive Enclosure and replaced the plastic positioner you should attach the drive to the enclosure using two of the provided phillips screws. Two holes on the underside of the enclosure should align with two attachment holes on the underside of the drive.