I’m in agreement with everything except the wording of “investigate and prosecute”, which makes it sounds like prosecution is a foregone (rather than a likely) conclusion, which would then diminish the integrity of the investigation.
I would word it thus:
My justice department would actively investigate the IRS corruption under the Obama Administration and the questions surrounding emails and Benghazi involving Hillary Clinton and some of her associates. And as no one should above the law, any required prosecutions of those who have clearly violated it would proceed.
The problem is that even saying he will investigate Hillary and Obama means his chances of surviving until the election are pretty low.
Between getting Arkansas flu or Chicago dengue fever, he’s going to have a tough time. I’d forecast a mysterious campaign bus crash in his future if Trump makes this promise. (Especially since the RNC will be working towards the same accident.)