I was thinking that you could have a fake out h-scene, where one of the more suspicious girls (perhaps after you've bimbo'd half their friends without building her relationship beyond a "hello") tries to seduce you. If you fall for it, she'd become indignant/furious, and unavailable to spend time with until you manually increase her bimbo to loosen her up. This might be something to implement much later, once the main paths are set in place.
As for when to normally get lewd, I'd say after or right before successfully maxing out their s-link, since they'd want to open up to the person that helped them so much. Maybe for the first h-scene you can "convince" their bimbo level up slightly? If they're bimbo'd first or early, maybe you can start h-scenes halfway through the s-link, as a way to get their bubbly minds off their troubles, although it'd take the place of actually progressing the relationship as a price (i.e. no progress bonuses from the conversation).