(1) Sure, a simple execute() could do, as described in the tutorial, which I recommeded to readers. I wrote a simple task that has a parallel structure to the more complex Jing task, following the guidelines in the first six steps of "Writing Your Own Task". Are those guidelines incorrect or out of date? If so, this section of the Ant manual needs to be updated.
(2) Ant is a nice tool, but I don't feel it is necessary to use it to build something as simple as Add.java -- one very small class with almost zero potential for continuous debugging or rebuilding.
(3) As far as using .class files: How exactly do you do that? I tried it every which way from Tuesday with no luck. Show us! (I haven't paid any attention to Antlib yet. Thanks for pointing it out.)