Let me answer step by step
if the app stores data with localStorage or web SQL, and the user
switches to a different standard browser on their Android, will the
app be opened with the new browser and does that mean that the stored
data is unavailable?
The data is saved in the ‘cache'(its not exactly the cache) of the browser, so if you change the browser, or set the settings so that the default browser is removed or changed, the data will go.
If the user doesn’t use the app for a year (which in my case is a
realistic and not necessarily a bad scenario), will the data have
expired like a cookie, or maybe been pushed out of the browser’s
storage by a deluge of data from other apps?
No, the data will stay there no matter for how long it is not used. So even if you clear the browser cache, it will still be there.
Or will the data be destroyed even earlier, such as when: – user
visits another site in the browser – browser is manually closed –
browser process is killed or dies – etc
No, the data stays all right. 🙂
Or are localStorage and web SQL the kind of storage that you only
delete when (in Android) you go to Settings > Apps and actively remove
the user data associated with the app?
Yes, the data only goes if you either manually delete it from your app or you uninstall your app. It will stay in all other cases.
EDIT:
In the case of iOS, the OS will remove the data in the local storage when there is a shortage of memory in the device.