Thanks very much for your answers This is really helpful. A lot of the time, I can see what I think it is doing, but I'm not certain because I'm not still that familiar with the app so I may have missed something - getting things confirmed (or corrected) is really useful.
Re the first point about storing the offline data - we will look at choice as an example, thank you for the pointer.
Re the last one about deleting data - we hadn't spotted the delete option in activities! As a starting point we'll try to make our own plugins do the same.