1) Regarding the judiciary's effectiveness as a campaign issue, I agree that is not necessarily the issue most likely to galvanize liberals because its not as easily understood as any number of us-vs-them, black-and-white issues, though when you look at any issue, you can find gray areas, abortion included. The question is, can Democrats effectively boil it down to "elect McCain and he will appointed bigots and religious nuts to the Supreme Court. Do we really want [insert name here] voting to overturn Roe v. Wade?"
Democrats historically haven't used this as a campaign issue, but they need to. Republicans since Reagan have aggressively done so, and it has paid off, but there's no reason it should resonate more on one side of the aisle than the other, aside from the fact that Republicans are used to hearing about it and Democrats aren't, to which I'd say that you've got to start somewhere. Fight them on it, rather than cede the issue. Yes, they've got a head start, but thats all the more reason to fight back.
2) Your argument that most people think that either the court is too liberal or that it is well balanced is true as far as it goes, but the opposite would also be true: if you add up those who think it is too conservative with those who think it is well balanced, that number drastically outnumbers those who think it is too liberal. McCain is arguing for moving the court to the right, so the more relevant metric, in fact, would be the total number opposed to it moving in that direction, which would be those who want it to stay in addition to those who want it to move to the left.
Furthermore, you write as if making the court more conservative and keeping it static were the same option, but they aren't. If one assumes that the court will change, the relevant question is in what direction do more people want it to move? Even if you assume that somehow we can keep the court as it currently is, McCain's desire to appoint more judges like Rehnquist, Alito and Roberts wouldn't accomplish this, it would move it to the right. Depending on who was leaving the court, replacing five of the seven current justices (excluding Alito and Roberts themselves) with another Alito or Roberts would move the court to the right. More people oppose this than support it, based just on the liberal/conservative metric, leaving aside those who wish the court to stay where it is.
3) When you discuss Kelo, it seems that you're willing to set that section of McCain's speech aside, but it is one of only three cases that he mentions at all. It is one-third of his actual discussion of court cases. If it were one of 20 or 30 or 40, saying "ok, what he says there doesn't make sense, but let's look at everything else" would be more justifiable, but it isn't. He highlighted that case in particular. He could have avoided it altogether, as there was nothing forcing him to bring that case up, but he chose not to. I focused on Kelo because I know that case well and his complaint against it is prima facie based on its result, not its internal logic, which you seem to agree with. While I can't definitively say the Court was correct in the other cases he criticizes, what's relevant is that his criticism has nothing to do with the legal basis for the decisions and has everything to do with their outcomes. You ignore the larger critique here.
4) Regarding Gewirtz/Golder, you mischarachterize what they say when you write "First, as Gewirtz and Golder admit, just because the Justices strike down (or vote against) a piece of legislation does not mean it is judicial activism -- the legislation itself could be unconstitutional." This is incorrect. They define activism as the judiciary overruling the legislature. They say that, in any given case, judicial activism may not be bad and that if a law is unconstitutional, activism by the judiciary is merited.
5) In responding to both Gewirtz/Golder and Miles/Sunstein, you say that their definition of activism is incorrect, but you never an alternative definition. Why is their definition, which is essentially "judicial activism is the judge's propensity to overrule other branches of government, be they the executive (Miles/Sunstein), or the legislative (Gewirtz/Golder), a poor definition? I think it to be a fairly good one, but more relevantly, it is also the definition used by McCain in his speech.
6) Re the lack of a control, that we don't know if Congress was passing particularly egregious legislation, is interesting, but has a couple of problems:
a) Given their definition of "judicial activism," whether or not the laws were constitutional is irrelevant. They are not offering a normative argument that judicial activism is, always and everywhere, wrong. McCain is the proponent of the normative argument here, more or less espousing the same logic in his speech that Gewirtz/Golder uses, namely that, as democratically elected branches of government, the decisions of Congress and the executive have a presumption of legitimacy. If you argue that Congress and the executive branch have made unconstitutional decisions, and that the judiciary is right to overrule them, isn't this really an argument against McCain as well? It seems that it is in favor of judicial activism, and thus undercuts the entire point of his speech.
b) How, from a methodological perspective, can you fix this? What metric could you use to determine whether or not the laws passed are constitutional, other than the judiciary's ruling on this question?
7) As for your "do they vote conservative because they are biased or because their interpretation of the Constitution leads to more conservative rulings," it seems that it is a distinction without a difference. Unless you believe that justices regularly make decisions in bad faith, thinking that their own arguments are weak but that the result they want compels a certain decision, what is the difference between a conservative bias, and an interpretation that regularly leads to conservative rulings? I'd also point you towards (IMO) a provocative post and response at Balkanization along these lines here and here.
8)You're conclusion overstates your case. You write that "the conservative jurist movement is neither out of touch with Americans nor necessarily illogical and that McCain, even beyond the obvious political reasons, is not wrong to advocate his position," yet you never put forward an affirmative argument that substantiates this claim. Even if you believe that you've successfully refused my original post (a point with which I obviously disagree), that still leaves you with a lot of ground to cover because you can claim to have proven McCain correct.