Ok, SPOILERS ahead:
First of all, playing with pregnancy doesn't change the game all too much. It explores an ideal fantasy where you start a family with your girls and there's additional dialogue and (non-lewd) scenes, amounting to ~20 minutes of additional 'gameplay' in total. If you want to see it all, follow all paths and creampie as often as possible. All 7 girls can be impregnated, though not every instance is explored in detail. Carla's impregnation has the most detail, followed by Rebecca, then Judie, then Lauren. Iris, Jasmine and Aiko can only get impregnated in the epilogue harem ending montage.
(MAJOR SPOILERS)
The epilogue is a collection of images without dialogue. It's part of the harem ending, check the official walkthough. Additionally, you need to tell Asmodeus that you don't want to leave anyone behind and you need to follow the path of at least 2 girls, but the more the better. Grabbing the grimoire is irrelevant for this ending, the walkthrough is wrong about this.
