Who actually reported geth on Noveria?

Well, Udina's actual words are:


So it is possible to interpret this to mean just some general sightings in both systems rather than actually on the planet itself.

Just before Udina's line that I quoted above, Anderson does say:



So, I think one could interpret the situation to mean that some of those geth were seen while searching for clues about the Conduit around Noveria before their searching lead them onto the planet itself after learning about the Rachni discovery.  It still seems to be an odd way for Saren and Benezia to have learned about the Rachni since they were already investors in Binary Helix.

I agree the whole structure of the situation is weak.  If geth sightings are the major clue that Shepard is to be following, then all of side quests in the Armstrong Nebula (Tali's data quests) should have been elevated to be parts of a main mission quest.  I think probably Noveria should have been gated to open only after recruiting Liara, who could have then informed Shepard of her mother's whereabouts in some way; but of course, Bioware didn't want to limit the player to doing those quests in a specific order.

I agree also about Iallis and Ventralis.  In that case, I just generally wave it off as Benezia not informing everyone about everything in order to be able to play one against the other is it came down to that... i.e. being able to order, say, Iallis to kill Ventralis or vice versa if either one outlived their usefulness to her... but it's admittedly a weak excuse.