Solus not appearing in boot can be fixed by using efibootmgr. There are multiple threads on the forum, but essentially boil down to:
- install efibootmgr from software center
- run:
efibootmgr -c -d <disk> -p <partition> -l "\EFI\systemd\systemd-bootx64.efi" -L "Solus Boot Manager"
Fill in disk and partition of the boot partition you selected in the installer.
For the error which fails the actual installation; It may have to do with the partition being mounted and hence not alterable by the installer.
Try installing again, but check gparted between setting it up (all the options) and actually starting the install to the disk. Unmounting it in gparted at this time then enables the installer to succesfully finish.
Of course, both issues can have other causes as well. These are just the two issues I had with these symptoms.