I am the dev owning the identity feature.
To boost the preformance for high end machines, we introduce preallocation for identity value in 2012.
And this feature can be disabled by using TF 272 (then you will get the behaviour from 2008R2).
The identity properties are stored separately in metadata.
If a value is used in identity and increment is called, then the new seed value will be set.
No operation, including Rollback, Failover, ..... can change the seed value except DBCC reseed.
Failover applies for the table object, but no the identity object.
So for failover, you can call checkpoint before manual failover, but you may see gap for unplanned cases.
If gap is a concern, then I suggest you to use TF 272.
For control manager shutdown, we have a fix for next verion (with another TF). This fix will take care of most control manager shutdown cases.