Enhanced Animations works well with GDK. It comes with some examples that shows pretty much the basics of what it is capable of.
1 : Enhanced Animation takes what you asked and runs with it, you can do more than just append you animation, you can take your animation and split it up, so you can apply say the bottom half of your idle animation and the top half of a dancing animation at the same time to the model, that would look weird I know but its just an example lol. EA comes with examples demonstrating this capability in much better ways than I just described. You can mix and match you animations to make them how you like basically, instead of being stuck playing animation1 from frame x to frame y.
2 : If the model can be loaded and you can play its animation with GDK then you can use it with Enhanced Animation.
3 : No, There is not IDE that comes with it, it allows as much control as you would have in an IDE though, but it does so from a programming interface, it would however be completely possible to build your own IDE using it, it has loading as saving commands for its own animation data once you have extracted it from your models.
4 : You can use any model that you can load into GDK with animation and have it play(ie with dbPlayObject) with Enhanced Animation, there is no need to create a "special Enahnced Animation format" model at all, it just works with your model via GDK.
5 : You can do that and alot more with Enhanced Animations. You can add extra keyframes and manually create entire animation sequences at run-time using EA commands inside GDK. You can alter the existing keyframes in their entirety, like I said, the functionality is there to essentaially build your own editor if you wanted to, allowing you to build a model and "skin" it in you modeller and load it into the editor you build, manipulate it with your editor and enhanced animation commands, then save the file out in EA's format, and load it up into your game.
I think it would be nice if we had an editor for use with Enhanced Animations. maybe oneday someone with some spare time will throw one together...
All in all, I think its a good product, and especially useful if you are going to need to customize animation inside you games or you really want some nice smooth and slick looking animations, as it will really allow you to improve on what you can do with GDK natively.
If it ain't broke.... DONT FIX IT !!!