Hello @emitorino @dclane ,
I have now dealt intensively with mount-control and identified the problem - unfortunately mount-control is not an option.
mount-control only gives me the option of using mount.* (in this case e.g. the mount.fuse3 command). Unfortunately, the library that zffmount uses (and indeed all other Rust libraries that are well maintained to use FUSE) using fusermount(3) in background (which is the current default way to deal with FUSE).
However, fusermount(3) seems not to be covered by mount-control, but only by the fuse-support interface.
However, the tool (zffmount) only makes sense if I can perform a FUSE mount anywhere in the system and not just within the snap directories - so the fuse-support interface only is not an option.
I have rewritten the whole tool (zffmount) again and also removed the nested mount, so that my allow_other or allow_root is no longer required - but unfortunately this does not solve the basic problem. 
I think I’m running into similar problems as restic would (which is classic and can only FUSE via fusermount).
The zff format is used to store data for forensic analyses.
In principle, it is a form of a backup - plus metadata.
As far as I read from this post here, backup tools are a way to perform a classic confinement…? 
thanks,
ph0llux