My daily W116 needs a hefty bang all the way home - on all four doors. The result is a solid "thunk". If one lets go of the handle before the door has properly shut, it will bounce back to the first stop position and require a re-opening and a fresh closing attempt. The weather seals are RS, so it's hardly those getting in the way. Whether sunroof and windows are open or closed makes absolutely no difference other than changing the tone of the "thunk".
The W124 300E that I've driven a lot recently has a similar door-closing style - though the required push is not quite as firm as the above car's. This car's weather seals are in good condition. The doors are harder to close if sunroof and all windows are shut tight.
Der Rolfwagen's (W116) doors - despite being very crusty (with RS weather seals) - close with only a gentle shove (but, as on all my Benzes, must be pushed all the way home, not left to rebound). State of windows makes no difference.
Crusty's (W116) doors - despite the car's being a parts-bin rustbucket of the first order - has doors that close properly with a gentle two-finger push. State of windows makes no difference.
On my '65 Finnie, all four doors shut with the lightest of touches (finger and thumb on the handle and the gentlest push home will do it). Weather seals: RS. State of windows makes no difference.
The very wide doors on my '65 coupe (with a driver's door that has dropped about 1-2 mm) work exactly as those on the Finnie - with weather seals in good condition. State of windows makes no difference.
My Ponton's doors - despite 29 years of inaction - also open easily and close with a very gentle two-finger push. Cannot comment on open/closed windows as sunroof is, for now, permanently "open" (i.e. not fitted).
My W108 had easily-closed doors until I had new weather seals installed after some rust work about a decade ago. Post-new-rubbers the doors are harder to close than on any other of my Benzes. They bounce back with a vengeance if not taken firmly all the way home. State of windows makes no difference.
My overall experience with Benz doors is that they are easy to close with one gentle push all the way home unless new (especially non-genuine) weatherseals have been fitted.
Although I've played around with latch-plate positions on most of my cars, it seems that if a door is inclined to rebound, nothing much will fix it; it seems to be in the nature of individual cars (every door on any car matching its companions).
Even if new and genuine weatherseals have been fitted, they may not have been fitted correctly - as evidenced on my W108 and, I suspect, daily W116.