User blog comment:Legionwrex/How did Saren reach the Citadel?/@comment-3228423-20130507041453/@comment-2250460-20130507042323

Even with Sovereign's implants, I highly doubt Saren would be able to travel through a Mass Relay and survive.

As to the Cerberus tip-off, I would assume that Cerberus just has a very effective spy network.