Just wanna disturb the great discussion between you and @muchen_, but just one observation looking the opening bar in your piece: in Bach's inventions either with long subject both voices begin at the beginning, or with short subject usually the other voice join after a bar or so. The opening sounds more like a fugue to me.
I think the 4# would work well if you modulate to C major for the answer, but you return to F major in the answer so it sounds a bit weird to me.
I think he's talking about the {C, C} in b.6 and then the {Bb, D} follows, which causes some ambigous harmonic progression to me as well since a bare C can indeed imply a V, altho I think you are going for an implied I 6/4 here.
for the D minor entrancr in b.8, you will prepare it better with C# before it.
Henry