With the main LIs, yes, the MC will be the only one involved. Side characters may have/develop other relationships (none of this is shown; the scenes will always contain the MC)
No plans for a harem, the game will at some point break off into defined relationship paths with the main LIs
NTR/sharing - as far as I'm aware, no, but some people define this differently as far as I've seen. For instance, there will be some options for scenes with multiple girls - if you class this is sharing, then yes. But there are no plans for scenes with multiple male characters. Similarly, lesbian content goes as far as scenes with multiple girls involved, but no exclusive girl on girl content
As much as possible everything will be avoidable, as in everything is predicated on the choices made, but there will, as far as I can envision, almost always be a final, implied 'yes/no' for a scene to start. So, you can make all the 'correct' choices to lead to a scene, but there will still be an option to say no, because it will presumably be possible to accidentally make all the right choices depending on how you interact with characters without intending on actively pursuing a girl
Hope this answers your questions well enough