Samara's recruitment is like
"here is the ship name, can we go now?"
"Sure, we're good, let's go"
Kasumi's recruitment is like
"I'll come with you if you help me with the heist"
"OK, I'll do that."
"Good, let's go"
There is difference here, dealing with Morinth is not part of the recruitment, stealing the graybox is. You are correct, not doing the LM is a good reason for her to resent Shepard, but doing it is not necessarily a reason for her to like him. Shepard keeps his part of the bargain, she keeps hers by helping in the Suicide Mission.
More like:
"You need to help me track Morinth by getting the ship name now and then I'll come with you."
vs.
"Promise you'll help me pull this heist at some point and I'll go with you now".
Again, don't look at it in terms of gameplay mission. Look at it in terms of personal goals. You need to do something for Samara's goal immediately. You only need to promise to help with Kasumi's. Actions are louder than words. And even there I'm wondering if there's a convo option loophole... The renegade option is "only if there's time". I've never picked it but depending on how that plays out, even the promise may not be what we remember.
I'd like to take this opportunity to point out the amount of roleplay options and choices we used to have, that are now vanished. Even if I agree with everything you say, it's still awful for being nailed to only one option (and a particularly distasteful one in my opinion), instead of several.
And while doing the heist is not the only reason for her to like him it's still a huge start. Like you said this is the one time she's comepletely open and he is there for her. Also there is the consideration that Cerberus made the deal, not him. Given how independent of Cerberus you can play Shepard the prior deal is almost meaningless. If your line is "I work with them, not for them", what they promised doesn't matter. You could tell her to screw off or tell her to go back to TIM and get other, actual Cerberus operatives for the heist. Or you could delegate to Miranda or Jacob, while you progress elsewhere. "They made the deal, not me. They cover it." Especially since they make no mention of it to you before you talk to her.
But you are the one who helps her, not just bust into Hock's and back out again but also with the greybox. You're the one that helps her keep a personal treasure or let go of the past and move on. And that is the start of a beautiful friendship. Or at least it can be. I don't argue my interpretation is the only correct one. But I firmly deny yours is the only one as well.
I never viewed Jacob "obsession" as something serious. For all we know that haiku could've simply been writing practice lol
Or, like you said, a rebound fling. I don't think it disproves "never again". I view the whole situation as an inner turmoil for her. On one hand, she wants to fill the gap that Keiji's death left. On the other hand, she keeps her distance not to feel the same pain again (especially as both she and Jacob are on a suicide mission). That conflict can go either way.
Consider, what do we learn from the room conversations about her personality? That she's sentimental, likes reading, cherishes memories of Keiji and doesn't like injustice. The first two can be learned by simply entering the room and seeing a lot of books and objects of sentimental value. The second two (and perhaps the first one) can be learned from the advertisement conversation. These are the personality traits she is willing to show to a complete stranger. I don't think those traits are worth to be secretive about, especially since the last two and the first are pretty much the reasons she gets Shepard in the Hock heist.
I don't see her as a shut-out person, but rather very secretive about her real self. A trait she has no doubt picked in her long career of thievery.
Oh I agree it's not serious. But even a crush/fling is enough disprove never again. And it does, even in your inner turmoil option. Which by the way, is not limited to two people
If Jacob, the everyman nobody can cause her to reconsider can Shepard do less? 
That's actually a way I can sort of justify the resentment of ME3. Shepard and Kasumi were close. But some kind of personal falling out occured. It's rough, but maybe I can pull it off.
What she reads can be telling. What art she enjoys can be as well. I am going full Thrawn here lol. I don't think you can read her like a book like he does. But there are clues. And the very act of strewing them about so openly is also a clue that she's not a shut-out. She is private, we agree on that. But she's also open to letting some people try to pierce the veil (no phrasing.) And the most important piece of information from those objects is that she doesn't stand for injustice. She sees someone that needs help, she acts. Which is ultimately why "I don't want to" doesn't fly in ME3. Again, she's not going to charge Harbinger like Grunt would. But she would be doing something, even if it is only helping refugees. If she had said "I can't come on the Normandy Shep, I'm not a soldier. I can't fight Reapers but I can rescue children from war zones and steal supplies from fatcat assholes who won't share to feed them" I'd be literally high-fiving the screen. No resentment, she's doing something on her own that prevents her from joining but that's ok, because everyone else is too. And both our visions would have room in that version.
I'll check it out, that's a content that needs to be missed IMO.
Agreed. Let me know if you find out before I do. Might take a while before I get to the party again, though I could reload a save...