To answer your question about the “permanency” of Element Ids. They are NOT supposed to be permanent. The idea is that Element Id of an Element in a WORKSHARED Revit model can change at any time during its life. To make a long story short the id can be reassigned when different people edit the element and then synch to Central model. Please see this post for more info: http://thebuildingcoder.typepad.com/blog/2015/02/understanding-the-use-of-the-uniqueid.html
Next time just use the UniqueId property, also commonly known as Guid. Those NEVER change.
Cheers!