Yes, you can use both localStorage and IndexedDB in your app but be aware of some limitations, especially during webxdc app simulation/development.
LocalStorage has a limit of 4-10MB which you can fill up quickly if not careful. IndexedDB is an alternative you can use that doesn't have this size limitation.
Note that browsers might reclaim storage for both localStorage and IndexedDB after a longer time of not using a webxdc app. If you want to safely persist data, you must send an application update which will be safely persisted by the messenger, and which also allows to use an app on multiple devices.
When you run your webxdc app with the hello simulator then all browser tabs share the same localStorage or indexedDB instance which is unlike when your webxdc app will be run by messengers. However, the webxdc-dev simulator manages to separate storage per webxdc app instance because each running app uses a separate localhost port for connecting to the webxdc-dev simulator server.
No, there is no guaranteed message delivery and also no feedback on delivery status. There is only a "best effort" approach. Messengers will typically queue messages and attempt delivery repeatedly.
If you want guarantees for peers receiving updates, you need to implement your own reliability protocol in your app. Common techniques include assigning sequence numbers or linked IDs to all updates you send, and implementing a way for receivers to request re-sending if updates are missing.
As with all "network synchronization" topics there are some theoretical limits. In particular it is useful to study and think about the Two-Generals problem and learn about existing "reliability layer" protocols before attempting to implement one yourself.