Brilliant Laura! Thanks for this. I'll have these answers for you. And of course if anyone else here feels like helping Laura out, please do!
One of the main reasons for a filter to 'break' is when any of the fields, issues, projects, etc it refers to no longer exists (or have changed in some way). The filter becomes 'invalid' and no longer returns any results.
Filters will always become 'invalid' from time to time as Jira evolves. It's just one of those things. There are two useful built-in scripts provided by the Scriptrunner app that can help with your other queries:
Change dashboard or filter ownership - This script will help you transfer ownership of filters from any leavers.
Switch to a different user - With this script, you could assume the identity of the leaver to see any filters that have not been shared.
Hope that helps :)
How do I decide between Cloud and Server?
(Yes, I know the answer, but a lot more people are asking, so I think it's worth a discussion!)
This topic is going to be discussed at our next meet-up, thanks to you for asking!