Mobile Discussion and Device Data Usage

Do you have an estimate of monthly data use on a phone plan of sending say 10 cases per week (with and without images)?

No, but we can estimate: a case is under 100k, and image probably doesn’t exceed 200k. The app does a download twice during a sync.

Can I only sync with a central instance on mobile?

The mobile client allows users to sync with either a central instance with an internet-accessible URL or with a local library instance, accessible over a private network via a locally-registered URL. The user chooses the URL which they want to use for syncing when logging in.