Macbook Pro 15” Late 2013 2.3Ghz. It was stupid of me to take this so lightly. I had a small partition that I had made in...

I finally got the disk to mount by using the terminal from the MacOS X and doing a zerodisk erase on the bootcamp, then also on the disk0s2 which failed but not due to dismount failure. Then I went into command + Recovery, disk02 had dismounted. Then I dismounted Recovery HD, and initiated disk erase from Disk Utility. Booted up from the bootable USB to High Sierra I had made and ran the installation. Everything installed perfectly. Now all is well :)