Theoretical isn't necessarily realistic but theoretical doesn't automatically mean unrealistic either, which you seem to imply. I don't need to suspend reality to make a reopening plausible if the situation I had mentioned occurred back then. If you added that many teams and dramatically reconfigured the conference, it's quite plausible there would be a reopening/renegotiation soon after. This isn't adding a team or 2, but 6 and in new markets. Same for a hypothetical ND addition. An unhappy long term partnership isn't something I'd expect any business would want, especially in the face of a dramatic change in circumstance like those hypothesized.
The only teams talked about now are G5 teams because the ACC has a GOR (none of the other P5 are plausible additions). Plus as I said to me the B12 is in a weaker position now so no one is leaving their P5 conference to go to the B12. I can't remember how Clemson got included but you mention the 2 were never shaky but IIRC it was FSU BoT member that started it all. It didn't just pop out of nowhere. I think there was even a rivals article about the FSU BoT member. Found the article.
https://floridastate.n.rivals.com/news/fsu-bot-chair-blasts-acc-opens-door-for-big-12
Haggard confirmed that as far as he knows there has been no contact between FSU and the Big 12 regarding possible expansion. However, he makes it very clear that he and the Board of Trustees would be more than open to exploring the possibility if it would mean additional revenue to the school.
"How do you not look into that option," asked Haggard. "On behalf of the Board of Trustees I can say that unanimously we would be in favor of seeing what the Big 12 might have to offer. We have to do what is in Florida State's best interest."
The FSU president was against leaving and he basically shut it down, plus the promise of an ACC network was later put on the table and that quieted things down. This isn't just some unsubstantiated internet rumor when you have comments from the chairman of the BoT of the school speculating about exploring options.
Even now you see articles again about how antsy some of the membership is about the lack of an ACC network thus far. Just think about what some might have been feeling a handful of years ago in the midst of all the realignment. Barring a B10/SEC pilfer, the ACC is in a stronger position than the B12 now so I don't expect any teams to realistically entertain leaving for the B12 like they might have back then.
As to making more sense for B12 teams to the ACC rather than vice versa back then. I don't see it as the ACC doesn't have room for enough B12 teams with them being at 14 already. They could take 2 and still try to keep it logistically manageable and those 2 would be western outposts similar to WVU. On the other hand, if the B12 had made a Larry Scott like play (they had room being 10 teams) back then they could have tried to get FSU/Clemson/GT/Miami/VT/NC State. So that along with WVU gets you 7 teams in the east and 9 in the west. A much more even break up and familiarity among the new faces. Maybe financially coax one of the smaller western teams to move east or maybe rotate those small teams on an annual basis. Point is you'd get a much more even geographic split this way as opposed to the other and they'd be coming along with many of their "brethren" without being on an island. It might never have come to fruition like Scott's play but the B12 never gave it an earnest attempt.