The Rise Of Neverwinter

I have some questions about the Mana System.
1. Will the buff to the Glass Half Full feat be retroactive? I took it when it was only 50% increase.
2. Will there be warnings about Self-Implode about to happen?

Yes, it will work retroactively.

On the Self-Explode, there will be no warning for those further away, but of those closer they will get a warning that something is about to happen.