Manafort's Russian ties aren't only known to Trump. The entire Republican Party establishment is well-aware of his background. Manafort is a part of that establishment and he's close to HW Bush.

This isn't Trump using private information to his personal advantage. This is complicity within the entire Republican Party establishment.

It's more likely that the Republican party itself has ties to Russia. Manafort was probably a compromise between the Republican establishment and the Trump campaign when he won the nomination, seeing as he let Lewandowski go and Manafort was tasked with moderating Trump's image. It also was Manafort who allegedly pressured Trump into supporting Ryan.

Trump is probably not using the Presidency to get preferential business treatment in Russia. There are much much much much easier ways to get edge, even in Russia. If he is, it's a goal that is way out there on the periphery, likely not something on which he would base his choice of campaign chairman.