And to reply myself: I found a solution. It is presented in my post under another topic .
I'll repeat it here for your convenience:
I have found a tested and working solution for those, who cannot remove XFENCE.kext from /Library/StagedExtensions/Library/Extensions/ - even via Single User mode:
Delete the actual kext from /Library/Extensions/ first, then run "KextWizard" (downloadable at https://mac.softpedia.com/get/Utilities/Kext-Wizard.shtml) and enable both checkmarks under "System/Library/Extension, i.e. "Repair permission" and Rebuild cache" - then click "Execute".
This removes the stubborn XFENCE.kext from the cache (that /Library/StagedExtensions/Library/Extensions/ actually is) and keeps it from being activated over and over again.
I strongly believe that XFence is dead as Mr. Zdziarski is now working at Apple and regardless of the fact that F-Secure bought it from him to gain access to its technologies to be able to implement them into their endpoint sec. products, Apple is "replacing" XFence's features balanced with usability (and of course, because of that with significant limitations) built into the latest v. of macOS.