[Tactics]

The way I see it ...

Aerith in ending, who lives and felt by Nanaki, is Remake Aerith who survived the canon event thanks to Cloud effort, staged by Sephiroth.

Reminder that she told Cloud not to look into (fragmented) sky which is shared traits of all doomed timelines. Timeline where Aerith lives shouldn't exist.
Cloud can see her thanks to Sephiroth intentionally made him aware of multiple realities, Red can feel her as he have enhanced sensitivity to Lifestream (which Sephiroth affirmed to be the true form of "multiverses") following experiment performed on him.

Then why she encourages Cloud to keep going? Its not OOC for AU Aerith to do so regardless of what she knew about Black Materia.
Reminder that OG Aerith appears (the one talked by Marlene, who sleep in Zack timeline) and caught Sephiroth off-guard before dissolved into green aura.
Remake Aerith become more aware she shouldn't survive and Cloud have to move forward. Her story ends there, the best she can do to minimize deviation after visible to Cloud is to wait and pray.

If she following Cloud, it will be exactly what Sephiroth wanted.
Not only it helps him to seek OG Aerith without White Whispers, but also will convinced Cloud to distrust everyone who disagree with him that Aerith lives.
Sephiroth already build so many advantages (event interference that not ruin canon events, corrupting Whispers) that the best OG Aerith can do is sit still, praying so Cloud not betraying OG path.