Check "About this Mac" storage I suspect it`s due to local backups from TM, even if TM is set to off this can occur
Q-6
Did you try volume verification in Disk Utility? A volume corruption once caused an issue where deleted files weren't subtracting from the free space.
In there I have a backups category, but it says 16.1mb
That will just be versions of local documents etc. not what is consuming the space. Try OmniDiskSweeper it will scan the drive for space usage, however be careful what you delete, if unsure ask or don't touch...
Q-6
Thanks! I installed it and not sure what it's showing me. I did see that I have an 8gb iphone backup.
You never know when a hard drive is going to fail. This is why I keep multiple backups of my data on external and cloud drives.
I would start using Time Machine. This is one of the reasons I use Mac. I had my HD fail several times on a previous MBA (which is not saying something good about Apple hardware), but in each case I could get all back from TM. When the same thing happened to my wife's Windows computer, she lost a lot of important data. Turns out that Windows file history does NOT backup everything, only what it thinks is important.
Been there, done that, HD catastrophically failed on one of my Windows systems in the field, thanks to multiple redundancy I was able to switch to one of My Mac`s and continue. I still manually backup with SuperDuper, and I have a secure cloud service (point to point encryption) which ensures all working data is backed up and updated on all my systems in realtime.
Q-6