Restoring Boot Camp boot from Windows 10 using High Sierra?
SIP can be turned off on your Mac. SIP has nothing to do with encryption. You have to boot the Mac in Recovery mode in order to enter the csrutil disable command. Recovery mode can be booted from the internal recovery partition, from the internet or from a USB flash drive. You can also use third party tools, such as rEFInd, to disable SIP.

rEFIt is the predecessor to rEFInd boot manager. rEFIt is now obsolete with respect to the current macOS operating systems. rEFInd should only be installed to either a EFI, FAT or jhfs+ partition. rEFInd should not be installed in the macOS partition.

The most likely reason the Windows 10 recovery solution did not work would be the partitioning of the internal drive. Normally, your Mac should be using GPT partitioning with a full protected MBR. If the partitioning was changed from full protect to hybrid, then the mountvol S: /s command would return the error message The parameter is incorrect. Here, I made the assumption you installed Windows 10 using the Boot Camp Assistant. If you first installed Widows 7/8/8.1 then upgraded to Windows 10, then this answer may need to be altered.

The easiest way to check and update the partitioning is to use the gdisk command. You will need to download and install gdisk. Also to use gdisk from macOS, you will need to disable SIP. You can also use gdisk while booted in Recovery mode.

If you are out of free space in the Windows partition and you can access your files from macOS, then you might what to consider first backing up your files, deleting windows, and then reinstalling Windows into a larger partition. I say this because there is no practical way to increase the size of the Windows partition without using some third party tool. Before attempting this, you would want to backup your files anyway. So, you might as well remove Windows, increase the partition size and reinstall.