For some airlines, the separate tickets may be an issue with checked bags. Most airlines don't have an issue with this, however, so that part would be something to check with the specific airlines you are considering. Either way, your bags should be pulled if you don't make it onto an international flight and you would simply recheck it with the new airline (if it came to that). ZED tickets are always priced point-to-point so you should be fine when it comes to refunding the unused portion if you have to reroute. You don't lose much if you book separate tickets other than, possibly, some extra taxes that may apply to a one way ticket as opposed to a round trip.