If Diarium stops being developed can we still use and download the apps from the app stores?
There are no plans to stop development of the app or removing it from the app store. But I can't give you any guarantee - no app developer can do that.
If something were to happen to you, would the program still function as is, long term?
Yes, at least I hope so 🙂
Diarium does not depend on any servers that I would need to maintain or pay for, as an example.