Huge System Files, can't boot into safe mode

Hi all,

My 1 TB ssd shows as 875 GB used, almost 700 GB as 'system files'. When I look at the actual top level directories on the drive (Applications, Library, System and Users, added together) it's got about 310 GB actually used.

I backed up the drive this AM, and it shows as 677 GB, but 269.57 purgeable - using Carbon Copy Cloner. So this lower number makes me suspect that the system files aren't real.

I know the System files aren't supposed to be this big, so I was going to boot into Safe mode and check it there to reset. It won't boot to safe mode using the shift key. I tried getting to safe mode in terminal, and couldn't figure that out.

I booted to recovery mode, did a first aid on the boot drive, tried again to go to safe mode. Nope. I tried resetting the p-ram and trying safe mode again. Nope.

So I guess I have two issues - how do I get the system files down (I suspect it's artificially large), and how do I fix the safe mode thing?

I don't have file vault on, and I have 'medium security' set for the mac, and the ability to boot from an external drive (in case I have to boot from my backup)