Yeah, Belzeebub should think long and hard about this decision. Trying to appeal to both NTR and harem fans almost never works out. The main problem is that harems rely on absolute, unshakable trust and devotion. Most harem fans will be unwilling to play a game in which the possibility of an LI being with someone else exists. Even if it's 100% avoidable, just knowing that an LI would be willing to cheat on MC if the wrong choices are made, is enough to turn off most harem fans.
Also, having both routes means dividing development time between them. Basically, if you develop both routes, each one will only be about half an update. Since there is almost zero overlap between harem and NTR fans, both sides are going to complain about the low amount of content for their route per update. By trying to appeal to both audiences, you'll only end up disappointing all of your players.
I don't think any game has ever managed to combine NTR and harem(or just romance) without at least some bleed-through. Renpy makes it rather difficult to completely separate two stories in one game. Anytime the two routes share any common dialogue, bits and pieces of both routes are going to bleed into the narrative, which will effectively ruin both routes.
