These are all interesting ideas. They might be difficult to code, but I like the discussion.
For myself, playing a character who has used magic scrolls for real life years, my priest character is up to the point of two in three scroll uses normally working. If I have five scrolls that I'm about to use, I generally assume that two of them are not going to work. That's my experience with a well-trained character.
When I first started using them on my adventurer, the failure rate was significantly higher as I remember.
If characters are using large numbers of scrolls without failures, they are probably not beginners in spellcasting. Of course...the RNG could also just have been really favorable to them that day.
