Why you think natural=cliff is problematic? According to Tag:natural=cliff - OpenStreetMap Wiki mapping as point or path is possible. The main reason is that an starting point of the via ferrata, if tagged with additional information such as sport=, name= or description= an physical feature is needed, otherwise this will result in an Osmose issue. I don’t know of any via ferrata that doesn’t start on a rock face.
The description tag can be used to provide additional information about the related element to the end map user. Why this should be abusing? If used, it is possible not to use the name tag (because otherwise it will be displayed in many renderers in contrast to the ferrata itself) and still keep the information available.
Ans yes, the mapping of nearly vertical ferratas is problematic (although I don’t know of any via ferrata that is really only exactly vertical). In France, for example, there are some via ferratas where the outward and return routes on the same wall run a few meters above each other - this is only understandable if the upper route is slightly offset towards the rock. Because also the satellite images are not real vertical, I think this is no problem.
In my opinion, the use of a super relation is not necessary, as I have already written. This does not result in any additional information. The combination of several variants/routes at one location was previously only done because there was no detailed information on the individual routes (name, color) available.
In your example of Křivý šev I think no super relation is neccesary. You have two relations with names and/or colors, but the different difficulties are defined in the different ways in an relation. Renderers of via ferratas recognize this and display the individual difficulties of the parts!
By the way: The use of via_ferrata_scale=0 makes no sense! This level of difficulty does not exist in reality. If there is a safety rope there, then it is at least via_ferrata_scale=1 (=easy) and if not, then it is a path.