Trump can't be prosecuted for obstruction of justice while in office. He would have to be impeached and convicted by Congress, which would likely fail even with Democratic control of both houses of Congress. Two thirds of the Senate would be needed for removal from office, and the Dems will be lucky to take control at all. They are considered likely to retake the House, but impeaching Trump there won't help with the Senate. After Trump is out of office in 2020, he could be tried in a legal process for crimes committed while in office, although the Justice Department might decline to bring charges in order to spare the country a lot of drama and embarrassment. (I hope not, but whoever wins control of the White House probably won't want the distraction. Attention and focus will be needed to repair all of the damage done during Trump's toxic tenure.)
Also, the question of Manafort losing his 5th Amendment privilege is far from clear, since anything he says under oath could be used to incriminate him on state charges. It is likely that, if there is even the slightest possibility of Manafort weaseling out of giving testimony under oath, he'll go for it.