For me the plot makes more sense with incest patch, along others might disagree. But both work fine. Keep in mind though that the patch is outdated, it's for 0.6 and not for newer versions, so it might crash the game. The multi-mod does have the incest tag integrated though, and it's for 0.7. But the content is the same with or without incest, it just changes the plot a bit.
About the 2nd question, you can still romance every girl, but you won't be able to have see all content, there are some scenes that you get from one girl or another, or none, depending your choice. Which is sad, because of my favorite girls on this game are on the most notorious choice. But you still are able to have content with all of them in one playthrough, just not ALL content.
There isn't any confirmation yet of harem with all girls, and the answers given hint that we won't have that in the end game. But that may can change in the future, we never know. Still, even if I like to get all girls and not let one behind, I deeply recommend you to play it. Definitely in my top 5-6 on this website, it's well worth it, and the dev is actually a nice and down-to earth guy.