Do I need to do anything before going somewhere with no signal?
No. The app stores what it needs while connected. Simply having opened the app with a connection at some point in the day is enough.
Are photos stored offline too?
Yes, at full quality. They are compressed only at upload time, so no data is wasted.
How many visits can I record before syncing?
There is no imposed limit. In practice the limit is free device storage, and a visit with photos takes a few megabytes.
What if I uninstall the app with visits pending sync?
They are lost, like any local data in any app. It is worth checking the pending-sync counter is at zero before uninstalling or changing phone.
What if two people record visits to the same store offline?
You get two independent visits, both kept. There is no conflict to resolve, because a visit is a record of a moment rather than a shared document.
Does syncing use a lot of data?
Almost all the weight comes from photos, which are compressed on upload. A typical visit with ten photos lands in the low megabytes.