Citizens United is simply Freedom of Assembly + Freedom of Speech = Freedom of Speech for Assemblies of People.
Not quite. A corporation is not an assembly of people. By definition, it is an aggregation of capital. This is why you can have corporations that are entirely owned by another corporation, with the only human involved being the notary in another state who serves as the registered agent (at arm's length).
As you know from your thorough study of the writings of the Founders, especially the Federalist Papers, there was a strong belief that rights and responsibilities go hand in hand. A corporation is designed very specifically to remove personal accountability from shareholders (who are the owners of the corporation).
How can a legal mechanism be used on the one hand to shield individuals from personal responsibility be used at the same time to confer greater rights?
We'll look back on Citizens United approximately the same way we now look back on Brown v Board of Education. As the product of a shameful period in our history.