The original super PAC
http://www.workersvoice.org/splash/Following the SpeechNOW.org decision (which was really more important here than Citizens United), the AFL-CIO established Workers' Voice, an independent expenditures-only committee under the guidelines allowed in the SpeechNOW decision. As it is not connected with any candidate, it is not limited by the $5000 / person annual contribution limit, and may make ads about any issue it wants to, provided it never coordinates with or donates money to any campaign. In describing this new kind of PAC, Eliza Carney coined the phrase "super PAC". (AFL-CIO, incidentally, filed an amicus brief in support of Citizens United at the time, though it has since called for the decision to be overturned, while still running a super PAC.)
The alternatives to super PACs are the traditional PACs (which can only have members of a given business, union, or trade association as donors of up to $5000 each, and can donate to campaigns) or Leadership PACs (which can take donations up to $5000 from anyone, but cannot contribute to campaigns, but can coordinate with them). The super PAC is kind of a trade off from the Leadership PAC: on the one hand, the $5K per donor limit is gone; on the other hand, no coordination whatsoever with any campaign is allowed (so, incidentally, a candidate telling a super PAC to pull an offensive ad is illegal).
Citizens United is really not as simple as people want it to be. The case was about whether a filmmaker could show a film critical of Hillary Clinton just before the Democratic primaries. Actually, it wasn't even that -- he could always show the film; the question was whether advertising it counted as "electioneering" and so under the purview of the FEC. BCRA ("McCain-Feingold"
had only been in full effect for a couple of years at that point, and it forbade a corporation (which includes unions, as well) from mentioning any candidate in a broadcast within 60 days of an election. SCOTUS ruled that the advertisement for the film was legal, but during the period around the election the advertisers would have to disclose their funding (this isn't normally true for film advertisements). (Note that the filmmaker himself could have paid for the advertisement, but the film's LLC could not -- pretty much any commercial film has its own corporation.)
Stevens, Ginsberg, Breyer, and Sotomayor joined with Kennedy's majority opinion on the question of requiring the disclosure, and dissented on the remainder of the opinion ruling the advertisement not under the FEC's purview. Thomas did the opposite (he opposes any funding disclosure laws whatsoever). (Stevens also correctly argued that the petitioner had not asked that BCRA be found unconstitutional, so the court was wrong to do so.)
The SpeechNOW.org case, handled by the Court of Appeals, used Citizens United to set out the system we have now: any committee that wishes to make political expenditures but not coordinate with any campaign can register with the FEC as a super PAC, and any corporation or union can donate to those super PACs without limit (in practice, a union or trade association* forms the PAC and is indistinguishable from it in everything but name). Roughly, this is the same idea as "soft money" (direct donations to party committees), except that the money is controlled by the interest group directly rather than the party; in both cases they cannot directly be used for or against specific candidates. I think the larger issue here is that this is part of the decentralization we see in so many things today (kickstarter, Uber, DIY record labels, what have you), and that traditional power brokers are upset that they are losing control of things.
I'm not as happy about Citizens United as the ACLU is, or AFL was (and I'm not sure how sincere vs. cynical AFL's change of heart is here), but I have to admit there's some sense in ACLU's main argument that it's hard to think of a better way. Should the FEC be in charge of what books and movies get to be released during an election season? Why shouldn't the AFL have the ability to make media buys on any issue it cares about at any time?
* Trade association rather than a business directly because for tax reasons I don't remember it's much easier for a non-profit corporation to do this than a for-profit corporation.
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Unfortunately, the SCOTUS seems to fall for it often. Then again, for a lot of our history, the Court has been rightist. The Warren Court was the aberration, not the Rehnquist Court.
Disclosure is almost always a good thing. For when it is not, see NAACP v. Button, one of the cases I believe the Citizens' United court mentioned to show that two different lines of cases had developed over the years.
That said:
(1) While the OP contains some straw men and false equivalencies, drawing lines in this is not easy, even though courts draw lines all the time.
(2) Forget the Constitutional amendment to overrule Citizens. Really, forget it. Repeal is a pipe dream and is not going to take money out of politics anyway.
No controversial amendment has made it through Congress and ratification since the Eisenhower administration and Congress is more gridlocked than ever. Not to mention something as vanilla as the ERA could not make it through ratification.
Even if an amendment miraculously made it, that would get us back only to something like the 2008 Presidential when Obama raised something like .75 billion in hard money alone and McCain voluntarily proceeded under McCain Feingold--and even McCain violated McCain Feingold! If Feingold were to run for President, even he would probably not opt to proceed under McCain Feingold (tho' Sanders might). McCain Feingold is still alive and well for those who want to choose it voluntarily.
BTW, believe it or not, the worst part of Citizens was not the money in politics part. Politicians use that as a battle cry, but most of the crap Citizens allowed was already happening. The worst part that declared corporations to be people. That was indeed quite a departure from the status quo. It changed far, far more than did the political part. As years pass, we'll come to see just how far-reaching that bit was. 