I can't help but think Kalros dragging the Reaper underground would have terrible, terrible consequences. We already know destroyed Reapers can still indoctrinate. And Kalros is going to get pieces of Reaper in her body, whether from trying to eat it or from it leaving pieces in her. I'm pretty certain either of those would eventually kill, or completely take over, Kalros. Even if Kalros is left alone, any Krogan that ever settles near the site of the Reaper's destruction will be slowly enslaved by its will... after a long enough period of time and smart use of its indoctrinated Krogan, it could bring the entire planet under its sway--not necessarily all indoctrinated, but all working towards its ends. From there, taking advantage of Krogan reproduction rates over the course of several generations, it could direct the creation of a Reaper from Krogan bodies (the genophage was what disqualified the Krogan as a potential race for conversion, after all).
Once it does so, this lone Reaper (either a fully intelligent entity of its own accord, or a new body for the original Reaper to transfer its programming into) can leave and repeat the Collector's method of gathering bodies. By taking only most of the people in a colony, and indoctrinating the rest, it can then get servants inserted into galactic civilization, which would work to find positions allowing them to cover up the Reaper's actions. Over time, the Reaper would be able to create at least one more Reaper, which would allow more daring action. By picking off colonies and retreating when confronted with large forces, they would eventually be able to create a small force of Reapers. Utilizing their indoctrinated servant (and the following they would amass), the could instigate a galactic war, using the chaos to pick off larger planets for faster collection. By carefully managing and extending the war as long as possible, they could wear down at the military forces of each side (attacking production sites and blaming another faction would accelerate this). At the lowest point of the war, they act to take out the remaining military forces, and as each Reaper is easily worth a goodly number of other vessels, this would be a battle the Reapers could win (especially since they'd use tactics other than absolute, overwhelming power like the Reapers normally do when they fight). With the military of the galaxy largely destroyed, most of the force would rest in groups like pirates and other criminals with money. They are most definitely not going to work together well, allowing the Reapers to go harvest highly populated planets with little fear of repercussion (although they would still have to do brief, routine checks to ensure the rest of the galaxy isn't getting its shit together and making more weapons). Take the stealthy approach for another three or four cycles, and they'd have enough numbers that the chance of even a ready-for-war galaxy would have little chance of stopping them.
Of course, any of the three not-useless-suicide endings would invalidate this... but from their knowledge of what the Crucible was meant to do, they had no way to know that the Reaper would be affected inside a planet.