[rbtying]

1. We thought about this at one point, IIRC, and various parts of Dropbox sync have been formally modeled. But CanopyCheck and Trinity are useful in part because they test the real, production code -- the hash-indexing bug that Sujay mentioned elsewhere on this page is an implementation error, not a design error.

2. Kind of. There are tests for interactions between instances (in particular, between different Dropbox folders on the same machine), but running Dropbox twice on the same folder is explicitly not supported.

3. Yes, this happens. There's logic to handle these changes, since some applications expect to use in-place edits and others swap in a new file -- you don't actually need grace to have an interesting situation. You can experiment with this yourself in a Dropbox folder :)