You most likely have Solus installed while the bios was set to UEFI mode. This will install Solus with the clr-boot-manager bootloader and not grub. Hence there are no grub entries created and hence Mint Grub will not have the entries available to boot. Reinstall Solus while the BIOS is set to 'BIOS Legacy' to install Solus with grub boot entries.
The fact that the entry is located in loader/entries and the file extension is conf shows that it is not a grub entry. Grub uses .csv files in the same folder as the .efi.
For example, my fedora grub files are like this:
While my Solus boot files are:
And yes, I think the partitions/directories you list will result in a complete removal when all are removed. Removing boot directories may be harder than removing the partitions though.
Consider using the BIOS itself to choose an OS to load (and the bootloader for which version, LTS or current, to boot) instead of one single bootloader for everything. In my experience that will save a lot of trouble when messing around.
disclaimer: This is all based on the notes I have taken at some point during my own experimentation. It may have some mistakes or misinterpretations of how it actually works.