For sake of argument lets accept everything up to the final push and harbinger's death beam(I’m going to assume that he made it through unscathed). 1) We know that this conduit went to the citadel, and we didn’t see his squad die; therefore we know Shepard is now on the citadel with his squad. 2) We know that Shepard needs to open the arms for the crucible to dock, and(from ME1) that there is a master control panel both in the citadel tower and C-SEC HQ; therefore we know our objective. 3) We know that the reapers had been transporting humans both dead and alive up to the citadel for processing, as well as the fact that the citadel itself had millions of inhabitants; therefore we know that there are both enemy forces on the citadel as well as captives. 4) We learn from Hackett after PRIORITY: CITADEL II that the crucible has the power to destroy the reapers but that the catalyst is required to activate it, as well as from the Henry Lawson recordings on PRIORITY: HORIZON that it is possible to control the reapers; therefore we know that the end result of this mission includes either the destruction or control of the reapers in some form. 5) We know as a result of Shepard’s conversation with TIM before fighting Kai Leng in PRIORITY: CERBERUS HQ that TIM “has what Shepard has been searching for”(the catalyst), and as revealed by Vendetta 30 seconds later that the catalyst is the citadel; therefore we know that TIM has taken control of and is onboard the citadel and is the final boss fight(likely including waves of reaper troops because they are onboard and he can control them). 6) we know via the war asset terminal that the collection of various war assets impact the end of the game, and via PRIORITY: EARTH mission parameters that the assembled fleet is broken up into those attacking the reaper armada and those defending the crucible; therefore we know(it can be reasonably assumed that) the end result of Shepard’s actions throughout the game involve the effectiveness of the sword fleet against the reapers based on appropriate collected fleet values, the effectiveness of the shield fleet against the reapers based on appropriate collected fleet values, and the effectiveness of the crucible itself as a result of appropriate collected crucible values.
In summation, and based on deductively reasoned evidence from ME3; the final mission after getting past Harbinger and up to the Citadel involves fighting with your squad against reapers until you reach the master control terminal and then battle The Illusive Man. After defeating the Illusive Man the crucible will or will not dock with the citadel depending on your collected fleet war assets, at which point again, depending on the total collected fleet war assets, the sword fleet either will or will not be victorious on its own and the reapers may or may not win. If there are sufficient fleet war assets to dock the crucible, then Shepard will(because they always have an opinion) discuss the proper course of action with his squad and chose to either destroy the reapers or use whatever modification TIM had made in order to control the reapers. At this point, based on the total collected value of crucible related war assets, anything from a galaxy destroying supernova(epic fail) to the targeted destruction of only the reapers themselves(epic win) or galaxy wide indoctrination of all organic life(epic fail)to total control of all the reapers(epic win) will happen.
Ultimately, while this is essentially still an A/B/C ending in which the reapers are either victorious, destroyed, or controlled, it is consistent with and deductively reasoned from the facts and plot elements provided throughout the game and therefore a more logical ending a better ending that want was provided by Casey Hudson and Mac Walters.
This took me 30min to conceptualize and write down. Also, it doesn’t have the starchild so it is automatically better.