I think 2 and 3 are already there and 1 can be faked with a splash effect killing the unit too
Just 4 is not possible yet.
If that is so, then it appears you have quite a lot of undocumented code. According to the thread in which that feature was documented upon release, you can specify the unit and the number, that's it. Number 2 should not be confused with just a negative HP regen. That's far too limited (eg, we may want a unit that only dies on its own accord, not something that could easily be struck down by the target before it even attacks, or suppose there was some type of healing going on, or if the spawned unit was the exact same as another regularly produced and used unit).
Number 3 you only have half of. You obviously don't have any choices for how to deal with the what to do when you reach this limit. Instead, the engine chooses for the modder.
As for number 4, that's really the most important of them. The rest of them are "nice-to-haves" that will enhance gameplay, but it's really number 4 that is the make-it-or-break-it decider, as seen by a number of people reporting the spawn attacks "do not work".