The viewer was told via channeling that this was a replacement fetus, brought into being to match to the original timeline and make it appear that the attempt to disrupt the pregnancy failed. If so, why was that necessary, as it would seem to arouse more suspicion?

While that would seem to be so, the extraterrestrials do not always succeed in their manipulations, and this is true of causing miscarriage to take place. That is often blocked by divine intervention largely through prayer requests. So while this is annoying to them, they would not see it as something extraordinary, because of the prior experiences, with multiple attempts. We wanted to have things further along very quickly at the outset because that is easier to protect than the very early stages should there be an energetic manipulation to terminate the pregnancy yet again. In a sense, having put this couple through the pangs and agony of assuming a miscarriage had happened yet again, the interlopers have the consolation prize of having inflicted great pain, even if they do not succeed from their recent attempt to terminate the pregnancy, and that works to the advantage of the couple here in our efforts for them to escape close scrutiny and earnest follow-up to compound the damage.