Erasing all the traces - I explained. So species would develop naturally - without artifacts. Which tend to portray Reapers as REAPERS. They are but they are for the better good - revealing themselves early thru artifacts - would ruin the cycle. Because species would focus on defeating them rather than on the problem. Also natural development leads them to relays and then to Citadel - the "mouse trap". This is the plan. Imagine species finding "Liara's message" and breaking the cycle - the galaxy would be doomed.
Second, It's not "we kill you because you get killed anyway" - it's "we kill you to save the galaxy". "you exist because we allow it - you will die because we demand it". Reapers sacrificed themselves to allow new species to emerge. They surely do not need to explain themselves why they are exterminating the failure species.
There are no holes whatsoever. The whole thing is pretty logical. Cultivate new species, if they fail, erase them and cultivate new. Save the most potential species though and add them to the "science lab". The search for the solution was on two fronts: reapers in the "science lab" and new species. Each cycle increases the chances to find a solution. If it is at all possible.
As for reaper-larva - that doesn't change a thing.
They erased species for two purposes - preserving the purity of new species and making it more easier to cull the failed ones. They are not stupid. Just because one of the purposes is present in both scenarious - doesn't mean shit. In fact "easier culling" purpose is more fitting the "Dark Energy" scenario - because in "Starchild" scenario reapers are not limited in time - they could've just combed the galaxy checking in every square light-millisecond and kill everyone - if they wanted to. They could've just orbited the habitable planets - you know, I mentioned it in last post. They couldv'e go the Ur-Quan way from Star Control 2. And Shield all the planets with life. So even if synthetics are developed - they would be automatically quarantined and then destroyed by reapers. Does "Starchild" scenario still make sense to you? If not think about this: take reapers goal and devise YOUR solution. Is it the same as Starchild's? Couldn't you come up with BETTER one?
P.S. Consider Starchild scenario and answer these questions:
1. Why did Sovereign need to WAKE the Reapers? Don't they have like an ALARM CLOCK set to 50000 years?
2. Why Reapers needed keepers to wake them? Look p.1.
3. Reapers failed to prevent Geth, why? Why didn't Sovereign destroy them? Why did he took control of them instead?
4. Javik in ME3 tells a tale of their own Synthetics and the war which Proteans won. Where the fuck were Reapers? Why didn't they give that cycle another chance?
5. Why not a peaceful solution? Like "Hi Hunams/Arasis/Tuniars/Saralians/Whatever, don't build AI, please. thank you in advance".
6. Why reapers sleep in dark space? Why do they need to limit the usage of Dark Energy Tech of their Cores? Boggles the mind.