> "The United States shall guarantee to every State in this Union a Republican Form of Government, ..."

Perhaps it’s my imperfect understanding of English here, but to me this reads more like a guarantee to the states that the federal government itself will take the form of a republican government, not that the states themselves must be republics.

It’s like a shopkeeper saying “My store shall guarantee to every shopper prices that are no higher than that of a competitor’s.” The guarantee is on the shopkeeper’s part; no obligation is required of the shoppers themselves.