Zeezrom, what honest act would hurt the Church the most depends on what you mean by "hurt." Hurt the Church among what audience?
I think what would hurt the Church about disclosing its finances is people seeing what the money is really spent on. I don't mean that I think there is a bunch of embezzlement or self-dealing going on. I mean how much is spent on salaries, fringe benefits, legal fees, public relations, business investments, etc. In other words, the things that look more corporate than ecclesiastical.
But where, exactly, would that hurt the Church? Probably in the same place the it would hurt the Church with other honest acts like not being coy about whether Jesus comes to visit the Bretheren, or presenting actual history instead of the sanitized faith-promoting narrative, or inventing arbitrary Mormon social taboos and declaring them to be "inspired counsel" that will keep the members "safe." It would hurt the Church's overall public image, it would hurt what hope for the missionary effort is still limping along, and it would hurt the faith of members who have not predetermined that the Church will always be true at any and all costs.
What is currently hurting the Church---and will continue to hurt the Church---is apathy among its longstanding members. Not an apathy from laziness, but an apathy because investment and reward are completely disproportionate in the Church. Aristotle Smith recently, and succinctly, summarized this problem:
Aristotle Smith wrote: Even more than fraud is the whole issue of relationship being a two way thing. If he is going to use the marriage analogy, then the church member has to be in partnership with the church. In other words, the church has to be some positive force. And there is the problem for Mormonism, when you get right down to it, it does very little from an institutional standpoint to nurture this relationship. Bland uninspiring general conferences, bland uninspiring manuals, no theological depth in anything, hundreds of rules, and defining a spirituality that largely consists in giving people a large list of things to do and making them feel guilty about not doing them when they inevitably fail. There are some positives about the church, but those are almost entirely the social/community aspects at the local level that haven't been killed by institutional rules and procedures, yet.
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So ultimately, I think what the Church could do that would hurt itself the most is to keep being itself. That is what is causing it to hemorrhage formerly active, believing members. The problems with it history are just the cherry on top of the sundae (and I will continue to mix metaphors until I run out of them!). Members who stop going, contributing, and caring because of the problem Aristotle Smith summarizes do not lose their faith because of the origins of the Church, but because of where the Church is now. For example, me.
But you are always going to have some members for whom it will not matter what the Church does.
"The Church is true!!!" in such cases is the beginning, middle, and end of how they will perceive and interpret anything and everything that happens.
One broad category is the devout TBM, which is a euphemistic way of describing a mild-mannered fanatic. No matter what the Church or its leaders do, it is evidence of the truthfulness of the Church (perforce, since it is a foregone conclusion). Look at the reaction to things like the Salamander Letter ("Well, the gift of discernment isn't perfect!") Paul H. Dunn being exposed as a fraud ("Well. they were still good stories!"). Among such people, nothing the Church could possibly do what hurt it. Thomas S. Monson could walk up to the podium at the Conference Center and start having sex with a goat, and they would be like, "Isn't it wonderful that the Lord is blessing the prophet so he can do things like that at his age?"
The other broad category is analogous to the Limited Geography Theory for the Book of Mormon (although "the" LGT is a misnomer, as there are in fact several versions of the LGT). It might be called the "Limited Church-is-True Theory." It is frequently found among Mopologists and/or militant cafeteria Mormons (the difference between the former and the latter is the latter don't usually feel obligated to invent crackpot theories to justify their selective apostasy). Much like the LGT, the Limited Church-is-True Theory ("LCCT") has as its objective shrinking down your expectations and claims of truth down so that it can't be falsified because it is too small to be measured. In the LCTT view, you can freely concede that there is little if anything miraculous about the modern LDS Church, and that many of the things the Church teaches as truths or inspired prophetic counsel are nonetheless false. No death before the Fall, the global flood, Joseph Smith having the slightest idea what he was doing when receiving revelation---all of these and more are out. Prophets as a rule speculating or giving their personal opinions as gospel truth, receiving personal revelation that Trump's that of living prophets and apostles, understanding what the Gospel is and what the scriptures mean better than the Bretheren or the Correlation Committee---these things are in. The problem with trying to define everything down so that it can't be falsified is that you surrender the ability to assert anything. Thus, you will never hear such a person explain what exactly they mean by "the Church is true," because they lack the meta-cognition to recognize that the only thing it means is brand loyalty. It has approximately the same deep spiritual import as Ford vs. Chevy or Coke vs. Pepsi.
But never mind any of this. We have the sure words of the prophets that the Church will continue to endure, and no unhallowed hand can stop this work. The mere existence of the Church is proof of its truthfulness, as indicated by this self-fulfilling prophecy. Never mind that any number of competing belief systems and denominations also continue to exist (and whose continuing existence presumably is not evidence of their respective truth values). So long as nothing happens that completely obliterates the LDS Church from the face of the Earth, and one single still lives who believes in it,
the Church is true!!!EDIT: I have noticed several typos in this post, but I am leaving them because this is how the wording appeared on the seer stone that reveals my Mormon Discussions posts to me.