Katie is definitely a serious relationship option. To me Jenna and Dani look completely different, similar glasses but Jenna has a prettier face and way better body.
No harem ending at least not a canon one. You might get a mini harem like Katie, Debbie and Lily. I could see a fake harem ending where the MC dreams he gets to keep them all, but in reality he's in a coma because all the women beat the shit out of him. But I'll keep pursuing them all until the beating begins. The start at any chapter option will come in real handy at that point.