Yeah, Chapter 2 seems set for 14 updates, again.
The "build up" is not really required. What has happened in DMD is that D has acted completely out of character (judged by the first few updates) to allow the plot to progress. Most players, immersed in the male perspective, routinely pan her stupidity for this. But, truth be told, without D's implausible brain-farts, there would be no "story".
Extreme fetishes can easily happen with Elena initiating a 3-way hyper game similar to "Truth or Dare". Elena's participation is sufficient build up.
I don't fully agree that she's been out of character recently: the D persona has always been susceptible to suggestion and once she's acclimated to a situation has generally become open to being challenged towards the next set of relationship inhibitions to topple. We're seeing that now as before, merely at a different pace ... which I feel is one of the few, more natural aspects to an IRL relationship to be displayed in this game.
We saw a rather long buildup for her to become somewhat proactive about her interest in the MC's penis, for example - but, once she accepted that previously unrealized fascination and decided to simply reach out and take matters in her own hands, she consciously rationalized that it was not a huge accomplishment ... because she actually cared about him. That has been her consistent point all along: if they truly care about each other, what is the practical limit? While the MC has pressed in various and consistent soft ways to get into her pants, she has remained overly idealistic about their future while becoming more desiring of being a romantic couple. Like any fool in a first love, she keeps making natural excuses for moving the goalposts and falling for every one of his setups.
That doesn't make her realistic, but it does lend towards a consistent flow on which to measure her more recent actions.
For, even in Chapter 2 she has yet to understand what it might require to maintain a long-term relationship with the MC, believing that they are already natural with each other and highly compatible. She hasn't realized her view of the MC is still as a protective authority figure and related catalyst for adult-like things, someone with whom she feels increasingly safe to expose her budding expressions.
Which is why I was actually hoping for a Chapter 3 where they might separate when actually living together exposed their imbalances, then she could try relationships with peers and they might eventually find a way back to each other on potentially more equal footings to start over by chapter's end.
As for her recent proactiveness, that seems consistent with her prior displays of accepting what has happened before and moving the goalposts because she's falling increasingly faster down the sexual exploration hill. In that respect, she actually resembles IRL young couples, I feel: once they pass a few bases, sky's the f*cking limit. They haven't matured necessarily, but they suddenly imitate what they think is mature by having lots of sex, in all different places and ways.
That's not to say you are wrong about her early odd choices, which certainly did make the current tone of the story possible - her level of naivete and positive responsiveness to MC's hopeful manipulations was entirely unrealistic and forms the basis for the relatively simple MC/D storyline, as I implied earlier. That doesn't stop me from enjoying her hyper-fictional innocence diving deeper into a physical/princess-meets-prince phase, which I have experienced and find rather fun to vicariously see acted here with a different setup, but same level of gusto and drive to fool yourself into wanting more "for the relationship" when it's anything but the basis of one.