So under 10 range, green cover or no cover is the same?
Point blank mechanism applies mostly to small arms. So green cover works for certain weapons even bellow 10 meters.
But Red cover still applies a debuff?
red "cover" does apply. Red cover is more like a property of the terrain and "less cover"
And trenches are garrison with green cover, do they behave like houses? Same question with bunkers
The are multipliers for green/garrison/trench/passenger are separate and different in most cases. They are close thou.
Keep in mind that "cover" provides mostly status and the modifier that apply are mostly a weapon property. Most weapon use similar modifier but there are some that use different ones.
In sort there is terrain, cover and garrison.