Could the M4 Sherman be outfitted with a 90 mm gun like the M36 Jackson?
Yes. Israel was one country which made such upgrades to their Shermans. The Argentinians even mounted 105mm guns on the M4 Sherman.
Could Michael Jackson ever be himself or he had to be the image people like a perfect performer the king etc?
King Jackson is a political slur used against Jackson in (I believe) the 1828 presidential election (although it could be 1832, when he ran for a second term) Jackson used his power to veto a lot during his term of office, and even as a senator he opposed bills. To make him sound like a tyrant, (such as the King of England) they called him King Jackson.