I know they both have feelings for each other but they are not on the same wave length. She seems to want to go into business and he wants her to focus on the wedding. If she truly wants to focus on business I think it would be good for the Duke to Support her. But he said he would invest and she did not want him to. I feel she is trying to delay the marriage. But I am unsure why since she asked for it. I am very confused about their feelings and what is going on. I will keep reading but I hope it is at least explained soon. If it is not.... (╯°Д °)╯╧╧.
It has a novel. You can read half of translated chapters in novelupdates(some chapters were summarised though, and the rest weren't translated).
And yeah there's otome game(kinda) for ISTMLFN in maybe interactive game, but only available in Korean. The app itself is available in the PlayStore though.
Even the none target ones. I am only on chapter 31, but I get the feeling she is one of those heroine characters that thinks all the good looking men belong to here. She may or may not be a Reincarnated character as well (not adding a spoiler since I have no idea). But original heroine characters in Villainess series who want to "catch them all" normally are reincarnated as well.