Here ya go From the "Jenova Relation 2: The Reunion" entry in the "Truth of FFVII" section of the guide, pages 210-213:
"Due to its amazing regeneration capabilities, Jenova's divided cells reunite to become one again.
5 years ago, Sephiroth fell into the Lifestream holding the head of Jenova, going to the Northern Crater, where the Planet's energy is concentrated, and where he would begin the Reunion -- and assuring his restoration -- by focusing through Jenova's cells. This regeneration was to be carried out by way of the Reunion, as the scattered Sephiroth Clones (-->P.213) would begin migrating toward the Northern Crater.
(Accompanying screenshot caption 1) The Reunion is an ability of Jenova, but Sephiroth instructs the Sephiroth Clones in the role of their master.
(Accompanying screenshot caption 2) The main body of Jenova began migrating from the Shin-Ra headquarters because of the Reunion. The Jenova cells in Cloud received the call of the Reunion the same as those of the mimicked form."
The entry makes it clear that Sephiroth initiated and controlled the Reunion, once in the main text ("where he would begin the Reunion"), and again in caption accompanying the first screenshot.
From the "Successful Work, Failure Work" entry in the "Truth of FFVII" section of the guide, pages 210-213:
"Also noteworthy is Sephiroth's unique case. Until the secret of his birth was known, he had legendary strength of which he was proud, but 5 years ago, he began walking a different road from that of humans. However, he was unwilling to be placed under Jenova's control and became the leader himself, with its actions falling under his control in the end. This fact resulted in Sephiroth gaining a new sense of strength."
The section covers Cloud and Zack first, but finishes with the above entry, which states clearly and in no uncertain terms that he refused to be controlled, and instead took control of Jenova.
From the "Sephiroth's will and Jenova's will" section of the guide, page 53:
"The Sephiroth clones seen in various locations continue gathering for the Reunion. Seemingly, the will of Jenova as a human is the result of it consuming Sephiroth's spirit; in actuality, Sephiroth is controlling Jenova."
