I'd put it a little differently. The Court acknowledged that it's inherently ambiguous to determine where a corporation "resides." But the question can't be dodged because the diversity jurisdiction statute requires courts to figure that out in order to decide if diversity exists. Lower courts had come up with a variety of tests; the Supreme Court decided to pick one of them so as to reduce the amount of ambiguity, while recognizing that it's not always going to lead to the "right" result.
The issue dealt with "principal place of business," and yes, lower courts had come up with a lot of tests. Justice Breyer goes through them in detail, and shows them to be a mess. One of the concerns was that California might be deemed to be the "principal place of business" for a lot of companies, merely because California has more people than any other state, so there are more business transactions there.
(In law school, one of the things first year law students study is something called "diversity jurisdiction"; that's what this case is about.)
This decision is going to have no real bearing on the issues you are raising. The problems of deciding when someone is acting on behalf of a corporation, when they are authorized to do so, and what rights shareholders have, have been around for centuries and are a matter of state law.
A Supreme Court decision interpreting the federal statute on diversity jurisdiction has pretty much no relevance to the state law corporate governance issues you're raising. It's certainly not binding precedent, and I don't think the issues are sufficiently related that any state court would find it to be useful guidance.
On this point, you and I disagree. I think "no relevance" is too strong.
I do not consider it a coincidence that this case and the
Citizens United case both delved into the nature of corporate fictions, and that this case came out after
Citizens United. Corporations have state existence (as you point out, corporations are creatures of state law [In Canada, by contrast, a corporation can be a creature of Canada or any province]), but there are many federal laws affecting corporate rights and obligations.
If a state were to determine that corporations incorporated under its laws could not issue a political ad unless approved by more than 50 percent of the voting stock, could the officers or directors of the corporation challenge that law on the grounds that the corporation's right to free speech (under the US Constitution) is being violated? I submit this is not an easy question.