Ok , now before anyone screams that 'FF-X isn't canon to FFVII' check wiki. While I know it's info is high subject at best there is a clear line between both 7 and 10.
Now heres my theory.
(Kinda spoilers for FF7 and Advent Children)
Sephiroths psychical form is gone , this we know. He returned in AC through Kadaj by manipulating the Jenova cells which (at the time) inhabited his body (plus Kadaj was a Sephiroth 'remnant' ).
Upon his defeat he remarks to Cloud 'I will...never be a memory'. Before leaving Kadaj's body.
From my POV it seemed that Sephiroth was (at that time) a form of unsent. Now before anyone tries to beat me with the canon stick there CAN be different forms of Unsent Shuyin proved this.
But let's cut the chatter.
Could Sephiroth ( Advent Children Form) Survive a sending ? Or would he fade into the Lifestream ?
FFX didnt have a lifestream.. the place where unsent went was another area / dimension of sorts almost like a prison.. had its own place.. in FFVII, lifestream 'was' the planets life force..it was all over the place.. but I do see the connection you have going
There is no 'clear line' between FFVII and FFX/X-2, it's just fans who can't take anything that has a similarity (as small and meaningless as it is) without connecting them.
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“You didn't kill him. He would have killed you, but you didn't kill him.”
“So? He was stupid. If I killed everyone who was stupid I wouldn't have time to sleep.”