The way Lena talks to Holly is, again, decided by the player's choices. Lena can encourage Holly to have sex, but she also has numerous options to (literally) "protect Holly" where she does the opposite and actively counters Ivy's ideas and influence. In fact, using these options Lena can form a monogamous relationship with Holly that's presented all like innocent unicorns and rainbows for both of them, including the supposed all-out-slut Lena.
Lena's former sex partners are also not set in stone -- she mentions having a lot of them if she's Lust oriented. She mentions having some casual partners if she's Charisma oriented. And she mentions none when she's Wits oriented which, given the alternatives, makes it a logical conclusion that on this path she didn't actually have many such relationships, if any at all. Lena is very much what you choose to make her and like i said, that includes Nerd Lena who'd kept very much to herself for most of her life, with her friendship with Ivy being more of "opposites attract" than "two peas in a sex pod".
As for the cuck route... the whole cuck route and talk about past dicks only happens if you choose for Lena to have sex with Axel in Ch.9 (or to work with Seymour against her misgivings) Afterwards, what Lena is doing is having sexy talks with her partner about things he clearly enjoys. Not only are these talks based mainly on what she's done in the game not before it (so yet, again, player's choices) but there's also no guarantee that what Lena tells Ian is 100% based on her actual experiences, as opposed to just telling him what he likes to hear.
I don't think anyone is saying that. I'm simply noting that the idea of Lena as "wild, adventurous girl with plenty of sex and someone Ian has no choice but to enter open relationship with and strive all the time to match her slutty attitude" is very much misguided and perhaps based more on how you've played your particular Lena, than anything else. She can be like that, yes, but she can also be nothing like that.
I agree with a lot of what you say, and yes you can make Lena behave very differently, depending on what choices you make. But that doesn't remove her former experiences, and remake her into a completely different person. Those continue to influence her behaviour, whatever path the player forces her down. She might be all lovey dovey with Holly, but once she's out of that situation, she often reverts to kind, particularly if you're also pursuing contrary content like Stalkfap or Seymour.
Yes, you can make it so she becomes just another Holly clone if you want, by avoiding every single choice, that instinctively you know she would likely make, if not for your input. Because at the end of the day, Eva wants to give players the agency to remake the MCs to their own tastes. She may have once been a nerdy girl before she met Ivy, but regardless of what brought them together, her friendship with Ivy changed her, and she became much more open, wilder and willing to experiment sexually. Come on, she's a nude model for god's sake, and she had a relationship with a guy like Axel. Would someone like Holly have done something like that, before she too came under Ivy's influence? Ivy has had a huge influence on Lena's personality, and whatever the player tries to do, that influence remains everytime the two of them get together
Yes, you might be correct that she's telling Ian what he wants to hear on the cuck route, but conversely she might also feel freer to reveal the complete truth about her former sexual experiences, because she's secretly enjoying tormenting him? Because lets face it, couples don't generally reveal everything about their relationships with former boyfriends or girlfriends, because it can often create awkwardness or even outright jealousy, particularly if in certain aspects they were better, than your current partner.
One of my ex-partners hated me talking about any former relationships, and got really jealous if I mentioned even the slightest reminiscence. So maybe in that situation Lena feels more relaxed about telling the truth about her former hook-ups (which often involved guys who were very different to Ian), than she is, in their more conventional relationships, where she's trying a lot harder not to hurt Ian?