Is Sephiroth actually Jenova?

[The Crystal]

The Sephiroth you followed around from the Shinra building up to that point was Jenova (working together with Sephiroth) using her(its?) ability to appear as another person. This is why when you first meet Sephiroth on the Junon boat he doesn't recognise you, as Jenova hasn't had access to that part of Sephiroth's memory yet. This also explains away the very weird alternative idea of Sephiroth going around carrying that great big lump of flesh.

You are wrong. Jenova is brain-dead during FFVII, she didn't have control over anything.
The reason for Sephiroth "not recognising Cloud" in the Shinra Cargo Ship, was because he wanted to screw with Cloud's mind.

She also morphed into Tifa briefly to get whoever was holding the black materia to come and hand it to Cloud, before turning back into "Sephiroth" and then disappearing.

Nope. Jenova's body was already destroyed at that point(after you kills Jenova Death). The Tifa and the Sephiroth you saw in that scene, were illusions created by Sephiroth.

It's a teamwork thing, they're both just as evil.

No, it's not a teamwork because Jenova is brain-dead. And because:

***Sephiroth's will and Jenova's will*** (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.

Jenova in FFVII is not a character, but a plot-device. Sephiroth used her cells to bring the clones to him(and the Black Materia), to regenerate his body, and to interact with the outside world(outside the Northern Crater).
We could say that Jenova is nothing more than an extension of Sephiroth.

Sephiroth has no speaking roles in the game aside from the flashback.

Of course he have. He is using Jenova's body(using his appearance) to interact with you, speak with you.