Well you could disengage from the monster and run away, then add the person, it only takes like 20 seconds to leave combat if everyone disengages. And monsters dont have instant regen, it takes awhile before they begin to regen hp back.
You would lose sp/exp for attacking it with less hp, but for experience mobs, its one kill, and for NMs you re generally looking for the drop over the exp anyhow.

When someone Dcs,
either kill the mob then add the person
disengage and run away then add the person and re attempt.

The connection lost on an NM is the only real reason its a problem imo, and disengaging while not perfect would work in that situation 99% of the time.

I do like the definitive answer (well as definitive as we re going to get anyhow) that was given, so now you can either take it or leave it, the system is as it is.