You can only talk Mordin down if both Wrex and Eve are dead. When playing Mass Effect 1, there was no way of me knowing that one day, Wrex would become the leader of the Krogan clans. Seriously, there was no evidence in ME1 to indicate that Wrex was interested in a position of leadership over the Krogan. If anything, he said he gave up all hope for his people. When playing Mass Effect 2, I couldn't have known that Mordin would suddenly go from being pro-genophage to actively trying to cure it. I chose to keep Wrex alive in Mass Effect 1 because he was a valuable ally. I did however, use the renegade persuasion to talk him down. So there is no inconsistency there. I kept him alive because he was valuable. In Mass Effect 2, I destroyed Maelon's data because my Shepard was pro-genophage. Especially since Mordin himself mentions in ME2 that the genophage is not equivalent to genocide but is rather a means to prevent a Krogan population explosion. EDI herself mentions how insane Krogan birth rates are in ME3 and how many problems it could cause. She says that if even 1% of 1 billion females become fertile, they could produce 10 billion infants in a year. Or something like that. Using the genophage was a rational decision to stop the Kograns from taking over the galaxy.
Now fast forward to Mass Effect 3 and suddenly, I'm trying to cure the genophage. I don't like it but I'm doing it because I need the Krogan as allies. The Salarians suddenly offered me a deal to sabotage the cure, get Salarian aid AND keep the Krogan as allies. The problem now is that even though Eve dies due to the lack of Maelon's data, Wrex is still alive. There is no way for me to talk down Mordin. My only decision is to shoot him. There is no inconsistency or failure there. My Shepard was entirely consistent the whole way through. Keeping Wrex alive in ME1 did not contradict anything. He was a useful ally. I don't hate the Kogran, i just don't trust them to form a stable government and to live peacefully with other species should they ever be cured of the genophage. Even in ME2, when you arrive at Tuchanka, you can hear some banter from two krogan who talk about how much they despise the Turians and Salarians. There is obviously alot of resentment among the Krogan against the people who attacked them with the genophage. How could Wrex possibly keep control of an entire species? Don't you remember Uvenk from ME2? Its obvious that not all clans support Wrex. Is it not possible that certain Krogan clans will break off and proceed to attack other species? The galaxy wouldn't be in a strong state to fight back and the Krogan would have the advantage of having an insane birth rate. Its completely plausible that a Krogan uprising could occur based on what we've seen of the Krogan in game and on their history.
Keep in mind that shooting Mordin is not only about sabotaging the cure but ALSO about getting Salarian aid. How is that not a rational choice? When the galaxy is on the brink of extinction, you need all the allies you can get. My Shepard was offered a deal to keep both the Salarians and the Krogan. That is a rational choice for the greater good. It isn't evil. If you think this is evil, then you might as well call the US evil for using the atomic bombs even though they ended the war with Japan.
Selling the Elven women to the Arl's son however is an act of greed and selfishness. You stormed the castle to save those women and now you suddenly turn around because the rapist offered you some coin? How could you possible think that that is more justified that shooting one friend and sabotaging the cure to gain crucial support for your war effort? You've already put the alienage in danger by attacking the castle. The Arl's son isn't exactly known for being an honourable man so how do you know he will just keep his word and forgive you? Its even possible that the Arl's son could convince his dad to be harsher on the elves based on what he suffered at your hands. Atleast getting rid of him would remove one powerful anti-elven political figure. We don't know much about the Arl himself. Atleast I don't remember anything. Is he as bad as his son? We have no reason to think he will suddenly destroy the alienage. You even see some soldiers coming to arrest the people responsible for storming the castle. They didn't look they were there to raze everything to the ground.
DAO. I am not selling anything. The Arl's son said: Take some money and leave. Take them with you. Them not razing the Alienage to the ground is the aftermath and you would not know when you are making that decision and even if they dont, the nobilities, the Arl will not take likely to lowly elf scum killing one of them, their son. More will follow.
See in your ME universe, you are the one who made all the choices that lead to that ultimate tragedy. In DAO, I am not responsible for how the nobles are treating elves. I am merely responding to things that others do.
My reasoning is, tactically the Salarians are not front line fighters and their are special ops and well that does not work well against Reapers. I can do without them. In ME1 I made the decision not to play God. I released the Rachni Queen because I am no Reaper and I will not punish people or commit genocide for what they may do. I then realize that there is a chance that the Rachni maybe a problem, so I supported a cure for the genophage. Need the Krogans to be alive and well just in case the Rachni come back and bite me; they didnt. Later I did everything to gain the Geth for more diversity and check and balance. 
Now, my universe has Rachni, Krogans and Geth. My idea of "check and balance" for what comes after the Reaper war and that decision was made way back in ME1, well the Geth was a plus in ME2 and I am glad all turned out well without me playing God, without me being a hypocrite fighting the Reapers when I actually supported their ideals 
So, as I say, in -my- universe, having to shoot Mordin from behind is unacceptable. 