[Lizard]

1) OmniFocus assumes that you're a single human being, so if an action is changed on 2 devices, it uses "last one wins" -- if you change a due date to Friday on your iPhone and then 10 minutes later change it to Thursday on your iPad, the due date will be Thursday. Changes are generally handled at the action level, so if you change an action's due date on one device, and its context on a different device before syncing, only one of those changes will survive. From your example above, the Mac changes would override the iPad changes.

The usual best practice is to sync each client as you start using it, and again when you finish using it. (In general the "when you finish using it" sync will happen automatically -- 2 minutes after your last change on the Mac, whenever you background the iOS app.) If you know you've got a device that's offline, try very hard not to edit the same things in both places (or make the whole action/project current, wherever you touch it last).

2) Mail Drop is specific to Omni Sync Server, because we have control of the server, so we can have this extra service running on it. Some folks use Reminders sync to let their SO send them "honey do" items. This does require those items to go through iCloud, which may be less secure than you prefer.

3) Apart from Mail Drop, syncing to Omni Sync Server and syncing to a different WebDAV server should work basically the same, because Omni Sync Server IS a WebDAV server. (If you wanted to for some convoluted reason, you could use the Advanced WebDAV UI to set up syncing to Omni Sync Server, manually type in the URL, and it would sync just the same.)