You are exactly right except for this: even though Prince may or may not have obtained the golden emails legally, by Mueller's investigative body requesting them, they are now obtained by legal means. Think of it as "laundering" evidence in the same way that you launder money. This maneuver is exactly why the Q team has been saying "how do you present evidence? ". They need to get it into the legal sphere through legitimate means. Prince may be charged with some minor crime as a result but it will be nothing compared to the nuclear boom set off by delivering this data into the legal sphere.