Now I'm starting to wonder the real mechanics of how RCV is adopted.
Firstly, what elections does it apply to? We know it applies to House elections, but it has been unclear if it applies to Senate and presidential ones too. Based on what you say here, it seems that it does, which gets into the how of its approval: via law or constitutional amendment? And which version is it: AV, IRV, or something else?
A constitutional amendment is almost certainly required IMO. Firstly, elections are something that are generally considered to be under the purview of the states, even the manner of how congressional elections are held (see: Alaska adopting top-four RCV runoffs, Maine adopting IRV, California and Washington doing a top-two primary, Louisiana doing a jungle primary, most southern states having runoffs). So to force a universal adoption of RCV, you probably need to force it constitutionally. Secondly, depending on how the amendments that adopted direct elections of senators and abolished the EC were phrased, this could also cause issues, although it's likely secondary to the whole "states run elections thing."
Now, gaining the required three-quarters of states does probably imply widespread support of the system, and I could see states adopting such election systems like Maine and Alaska did IRL as being something of an experiment that eventually takes off and results in a constitutional amendment (kind of like women's suffrage and prohibition). The thing is, I don't see an amendment that forces the states to adopt RCV, especially a specific kind of RCV, on the state level as being able to pass (too much of an infringement on states' rights), even in states that otherwise have adopted RCV themselves (and I could see a variance of which RCV system states adopt too, before the federally-adopted one is able to become the standard; hell, maybe even one state does a southern-style (heh), Bayh-Celler runoff system,). So IMO, even after the amendment is passed, you likely see states that opposed the amendment obstinately cling onto FPTP voting system they have (and even among some that do approve the amendment), even if over time the amount of states without RCV is trending downward.
TL;DR: I think it's likely at least one state refuses to operate on RCV at the state level, even if RCV at the federal level has been the norm for decades, and that some states do IRV while others do AV and so on and so forth, no matter which solution the federal level chooses.
