Mutiny in this case means as defined above. An old grudge is boiling over. Note that it is highly unlikely for the grudge to literally be ancient how I would think of that word today.
“Civil blood” Is a double meaning a reference to the prestige of the families and brings into contrast their incivility in their vendetta. Part of the magic of communication is deliberate double meaning. It is not a well defined API.
As someone with a disability who teaches computer science to students with disabilities (and without), your problems reading Shakespeare do not sound like those of people that come to mind when I read the term “regular person”.
If one is going to attempt to parse every word in the play and believes that it is the only way to follow the plot then one is going to have a bad time. If you could get to the Queen Mab speech reading that way then you’d probably melt at that point.
My suggested way forward is to not attempt to think definitionally about words that are used in a manner you cannot understand. When you arrive at a word like mutiny think that Shakespeare isn’t talking about a literal mutiny (though he might be) but something that is like a mutiny. What are mutinies like, not in cause but in deed. When Shakespeare starts talking about “atomies” he has no knowledge or Bohr or QM, but we know that atoms are really small. When Mercutio mentions Crickets bone… well, Crickets don’t have bones but if they did they would be small.
Shakespeare’s scribes weren’t writing an API for you to extract the plot out of. It’s a lot fuzzier than that.
[1] https://www.oed.com/dictionary/mutiny_n?tab=meaning_and_use