I understand your reasoning, but might I remind you that the whole of the Imperium is built on the foundation of blind faith and obeisance? Yes, back in the days when the Emperor still travelled the galaxy and lead his Crusade, he was promoting a completely secular world. However, since his entombement in the Golden Throne, things have changed significantly. If someone does not believe in His Divinity, they are going to be called heretics. So even though my words seem to suggest I'm just biased towards the Ecclesiarchy, I am simply referring to one of the defining core elements of WH40K: the Emperor is worshipped as a god and he is the ultimate authority. Therefore, doing anything that goes against the laws and tenets of the Imperium should invite corruption.
I understand that this is not exactly a completely objective look on things, especially considering how differently laws and faith can be and are being interpreted in the Imperium, but there are some basic things that should not be done. You don't deal with heretics, aliens and daemons.
Since you have brought up a lot of examples, I'm going to answer them, if you don't mind.
Philosophical discussion with the Tau Water Caste: The first mistake that you haven't killed the Tau the moment you met him, but since it has come to a discussion, especially a philosophical one, you are allowing the Tau's words to influence you. You don't have to accept anything he says, but his words will come back to you later and might change your actions in the future (perhaps sparing a Tau's life along the line), even though you are supposed to kill them simply for being xenos. Also, please note that humans who have joined the Tau Empire are considered heretics.
Should a purveyor and imbiber of slaught receive more corruption points than Warmaster Maccaroth sending 20,000 ill-prepared men to their deaths in order to stymie the political and military might of one of his upstart suboordinates? I believe this example may be a bit wrong. Yes, the slaught-lover should receive a whole bunch of Corruption points. And yes, Warmaster Maccaroth would also receive lots of Corruption for such an action. However, if Maccaroth had sent those people into the jaws of death against heretics, there would be no Corruption involved. It is because he used his power and authority for his own use and not for the Imperium that I would give him Corruption points.
Should a guardsman sharing water with the Eldar Guardian who just assisted him in slaying a handful of Orc receive corruption, but the Confessor who orders the purge of a thousand workers for little more than suspicion is immune to the corruption? Your first example would hardly come to pass, as the Eldar would probably shoot the Guardsman with the same dispassion with which he disposed of the Orks. But should such a scenario take place, yes, he should receive Corruption, although a very minor amount. And Confessor would receive Corruption points if he was wrong, but if there was indeed taint, his actions would be justified and no Corruption would be granted.
Is a Rogue Trader who has a writ signed by the Emperor himself immune to the effects of corruption for as long as he holds the writ? A Rogue Trader's writ doesn't grant him immunity to Corruption, he is simply given the authority to act outside regular Imperial laws and boundaries. It is up to the Rogue Trader how he acts and whom he consorts with. Those Rogue Traders who have dabbled into stuff that are against Imperial law are indeed afraid of the Imperial authorities because they know they are in the wrong and would receive a righteous kick in the rump for doing so. Of course, the Imperium overlooks some of their actions in favour of the other benefits the Rogue Trader's activites might produce, but a Rogue Trader can become Corrupted as easily as anyone else. He just has a paper that lets him do this more easily than other Imperial citizens.
Of course, this does not mean that my reasoning is valid when it comes to the game mechanics of Corruption. But bear in mind that even though there are clear rules (bit of an exaggeration), it is always up to the GM to decide how to interpret them and give players Corruption. The physical disfigurement part is hard to explain when it comes to xenos, I give you that. But doesn't mean, and this is strictly my humble opinion, that Corruption points shouldn't be given for actions outside the legal / moral system of the Imperium.