Harbinger saying "serve us" before hitting Shepard with the beam, which was added in the EC. As well as the extended dialogue with the God Child pretty much confirming he is a reaper and he speaks in Harbingers voice if you choose the Refusal ending.
Harbinger makes a generic Reaper sound effect and doesn't say anything. The IT crowd were hearing what they wanted to hear, similar to the people who see Jesus' face in the random shapes and colors on the surface of a piece of toast.
As well as the fact that there is no way for Shepard to survive Harbingers beam unless he was trying not to kill him, as it killed everyone else and cuts warships like butter.
The Lazarus Project was much more far-fetched than Shepard not being killed by Harbinger's beam. I think it is quite a stretch to see the latter as somehow proving IT.
and the God Child having the form of a human child that didn't exist at the time he was created, since synthetics cant read minds, Shepard was in a dreamlike sequence.
Shepard and Anderson had been under the effects of indoctrination minutes before, during the confrontation with the Illusive Man. Since the Reapers had indoctrinated TIM, its likely that is how the Catalyst was aware of the child and his psychological significance to Shepard.
That doesn't necessarily prove the entire end sequence was a dream however. In fact the Extended Cut suggests it wasn't with the epilogues where the end choices are revealed to have real results, and only end in failure and mass extinction when Shepard refuses to take any of the options mentioned by the Catalyst.