This is more out of curiousity than need-to-know:
Say the target has no DS on it. The -25 to its DS doesn't come back as a reverse DS that's then added to the reverse DS component of this spell, does it? The Reverse DS of that MOB would stil be just 45?
Also, if a MOB has, say, a DS of 70. If you cast this spell on it, while those hitting it be informed of receiving both life and damage per hit, will it not say anything, or will it combine them into one message, telling them that they've been healed for and damaged by 45?