Forum:A story question: Why Saren and why the Collectors?

Hallo there, this question might have already been answered but I haven't found any concise and illuminating response yet.

I keep asking myself, why Saren took such great pains to bring in the Reapers and why the Collectors abducted countless humans to produce a brand-new Reaper when the Reapers were already on their way to the Milky Way from the Dark Space? After completing both Mass Effects with all the DLC's, the plots in the two games do not make much of a sense considering the fact that the Reapers are going to arrive no matter what happens, i.e. no matter what the player did. In other words, letting Saren do his thing or allowing the Collectors to achieve their task, nothing would change for the worse... In ME1 we found out (on Ilos) that the Reapers are trapped outside the galaxy and the a help from within. They did not get this help, however, so why is their invasion still imminent? What are your opinions? Is there anyone who can provide me with a truly consistent and reasonable answer?