"Seductive" Daisy was already a rejected version when it was released the first day of the Early Access.

The Emperor existed from the get-go.
> BALDUR'S GATE 3 DEVELOPERS EXPLAIN ITS CONTROVERSIAL ENDINGS, BELOVED CHARACTERS, AND MAKING THE BEST RPG OF 2023 - IGN

Get-go? Likely ~07/09/2017 - First BG3's story drafted by then
> Eurogamer | Larian on signing Baldur's Gate 3, approaching a new era, and games you've never heard about

05/03/2020: While commenting on the BG3 cinematic released in February 2020, Larian's Lead Writer Adam Smith speaks directly to the game’s plot logic, outlining the Emperor’s narrative role without naming the character.
> Baldur's Gate 3 writer takes us behind the scenes of legendary RPG sequel

Voice actors: Ethan Reid is brought for the tone "trustworthy, very grounded with a kind authority".
Both actors recorded two versions of Daisy: the seductive persona used in EA, and the protective version ultimately delivered for release.
> Unofficial PlayStation Magazine | Interview w/ Ethan V Reid: The Voice behind Baldur's Gate 3's Dream Guardian

-> The 3 Daisy dreams got released day one of the EA but never got reworked (?)

Game files (EA and release) and the Artbook: “Daisy” is an internal label, a place holder name used to designate the dream interlocutor who appears to the infected characters during long rests.
In EA, interlocutor names didn't accompany dialogue lines displayed on screen; it's only since the late Patch #9 Daisy finally revealed as "Dream Visitor".

The Emperor was concealed. It was referred to as ‘the Companion’ during casting while a voice actor was being sought.
> EXCLUSIVE INTERVIEW: SCOTT JOSEPH TALKS BLOWING MINDS AS THE EMPEROR (AND MORE!) IN ‘BALDUR'S GATE 3' - NERDS AND BEYOND
(+ Some other twists were not in the Artbook despite the spoiler trigger, as for the Dark Urge.
> Eurogamer: The making of the Dark Urge in Baldur's Gate 3)

The "Dream Visitor" is already present in D&D lore, referring to "the Adversary", a former human who survived ceremorphosis, became a renegade illithid, and intruded into the dreams of chosen individuals to recruit them against mind flayer schemes.
> A Darkness Gathering

Down by the River was created as the main theme, not specifically for Daisy.
> Godisageek | Interview with Baldur's Gate 3 composer, Borislav Slavov

Daisy was not the tadpole.
The EA companions assume that Daisy is a manifestation of their tadpole since no alternative explanation is provided to them at that stage. But in both lore and BG3 (EA and Release), the tadpole is unable to articulate complex thoughts.
> Lords of Madness: The Book of Aberrations

All iterations of the Dream Visitor/Daisy share the same core function, only the tone of the dreams shifted.
> Parsed Dialogues (EA #5) nexusmods.com/baldursgate3/mods/17671
> Parsed Dialogues (Release, from roksik-dnd) tumblr.com/roksik-dnd/725479264465666048/bg3-parsed-dialogue-03082023-google-drive

Everything tends to indicate that the Dream Visitors have always been the Emperor.