Like most people in this thread (most, not all), I agree with all your points except for the one about how Liara becoming 'hardened' was good storytelling. Allow me to explain my point of view.
There seem to be two camps when it comes to this issue. There are those who think that it was poor writing/devs being lazy/devs trying too hard to make us hate the ME 1 LI's so we'd cheat/devs trying too hard to make the game 'dark' and 'edgy' (the camp that I'm in.) Then there are those who think Liara's 'development' was justified because she's been through hell/it's a facade she has to maintain/she's moved on from Shepard/she's in love with someone else (Feron or whatever he was called).
Firstly, I'd like to define what I think (this is my opinion of course, not fact) is meant by the word 'development'. Ironically, we need look no further than ME 2 itself to see fine examples of character developent. There's the two returning squadmates for a start.
Tali has obviously matured and become more worldy wise, so to speak, yet she is still very much the same person we loved/hated/were ambivalent to in ME 1. Despite thinking Shepard was dead and being interested in him in ME 1 (by her own admission if you romance her), she managed to remain pretty much the same optimistic, feisty character she always had been. Is this unrealistic? Not in the slightest.
Then there's Garrus. Again, he's certainly changed since ME 1, becoming a vigelante and, some would argue, more ruthless. Yet this is called 'devlopmet' and not 'completey altering' because it's really just an extension of who he was originally. He's always hated injustice and without someone to reign him in (Paragon Shepard in ME 1), he's prone to a certain degree of overzealousness. This is simply a realistic tweaking of his personality in response to Shepards death.
Wrex is another good example. Even though he cannot join you, the reasoning behind it is solid. In a way, he's doing what he's doing on Shepards behalf. After all, it was Shepard in ME 1 who called him on his attitude towards the Krogan people and perhaps persuaded him to re-evaluate his priorities.
Even the reunion with Ashley/Kaiden makes sense on a certain level. Although the fact they just brush Shepard off without giving him/her a chance to explain is a bit of a cop-out, especially if they were your LI, it is understandable why they do it. They specifically state that they're sticking to their ideal of being an alliance soldier, which is very much true to their character in ME 1. Whilst their stubborness does come off a tad ridiculous, especially when they trusted Shepard completely before, it's clear that they're still more or less who they were orginally. Even if it wasn't handled as well as the development of Garrus or Tali, they're development it still more of an adjustment rather than a total re-write.
Which of course, leads me to Liara. Just to make this clear, I have seen the 'hidden' dialouge with her, in fact, I got it on my first playthrough since I normally try to explore all the 'Investigate' options. What we see here is not 'development', at least not as I see it. 'Development' implies that the character has grown within themselves, that their personality had expanded and shifted based on certain events. What we see with Liara is a complete 180; a total rewrite of who she was before. That's not 'develpment'.
It's true that you would not expect her to be quite so innocent as she was in ME 1, after all that she went through with Shepard. That would be unrealistic, and I agree that she should have matured somewhat between the two games. Unfortunately, it appears Bioware interpretted 'matured' as 'turn into the polar opposite of what she was before by making her a cold-hearted murderer who's entire life revolves entirely around petty, unjustfied revenge'. Apart from that one 'hidden' line, there is no relation whatsoever between the new Liara and the old. People say she 'went through hell', but that's just a blatant exxaguration. What actually happen to her? She found your body, got attacked by agents of the Shadow Broker who was trying to steal it off her and eventually gave it to Cerberus so they could ressurect you. Since everyone else thought you were dead and she was the only one who knew you were coming back, she has even less reasons to move on than they do. As for being attacked, if you took her in your squad in ME 1, she's repeatedly put into life-threatening situations and extreme danger. Hell, she saw Shepard kill her mother right in front of her and aside from feeling a bit sad, it didn't affect her at all. The only 'hellish' thing she experienced was the guilt of turning Shepard over to a sworn enemy and I fail to see how that alone could cause such a massive morality shift.
In short, her 'developmet' is completely unrealistic and out of proportion to the 'hell' she went through. In reality, her experiences were no worse than anyone else's in the squad and they all came through with their personality intact. Aside from that one throwaway line, everything she's become is in such complete contradiction to who she was before that it's almost laughable. Even the comment about 'being unable to let you go' is irrelevant, since she says it even if you were insulting and horrible to her in ME 1.
Seeing her grow up a bit would be realistic. Seeing her conflicting emotions about whether she did the right thing would be realistic. Seeing her threatening to kill people, asking Shepard to pick out targets for her, ignoring any romanitc involvement with Shepard to the point of showing no emotion about his return whatsoever...that's not realistic, that's bullsh't.