Both Stripe and Nii are main quest characters, unlike side quest characters, they have their own animations. Side quest characters only have one image. I replayed one of the side quest to get the hint before the option, here is the hint, which means you can only unlock the scene by choosing the right option.
I don't know if, in the future version, you will be able to unlock the scene by talking to the character after the quest, but it indeed sucks a lot if you choose the wrong option and miss the scene. Hope they add an option to that, 'cause grinding in an adult game is pretty odd to me XD.