I know that the whole band wanted Portnoy back because he's back in the band
True, the details are unknown. Was JP wearing the others down to accept his own personal preference? did JP and JR equally wanted to work with Mike again and had to strongly convince Myung and LaBrie? was Myung on board with the idea and LaBrie had to be convinced? did they basically told JLB "look bro, we all want to do this, so just suck it up and accept it? of course we don't know.
We know the final outcome, that Portnoy is back, that means that "Dream Theater", as an entity, was willing to have him back. The discussions about it and how long it took them to get there, and the level of enthusiasm and convinction of the four individual members, are of course unknown, I don't know them, we can't know them, and I don't remember reading anyone being dead certain about this specific detail.
I take the decisions from the band as a sort of "royal we". We can wonder if the whole band is in full total agreement about anything, really - do they all equally want to tour in a specific time frame rather than two months earlier or later? do they all equally want to play a specific song? do they all equally like the cover art that Hugh Syme presents to them? when Dream Theater come out with these decisions, it's because "the band" collectively agreed to them.
If in the next tour Dream Theater will play Octavarium as the encore, would it be terribly wrong to say that "Dream Theater" wanted to play the song? can I say "wow, it's nice that Dream Theater finally wanted to play the song again" without having to question if maybe it was Portnoy alone that insisted to play the song, or maybe Portnoy and Rudess, or maybe just any other combination of 3 guys that wanted to play it and 2 guys that didn't care? the final outcome is that the song is in the setlist, so the final collective decision from "Dream Theater" is to play the song.