The body is going to flex. There's no way around it. The question is if the flex makes the latch stick.

The door latch pins are adjustable. If you want the door to open well then adjust them. With the car on the ground, move the latch pin down a bit while still having the doors open/close well.

 

The doors and hatch form part of the body structure (and stiffness).

When the car is lifted in a way that causes it to twist then the hatch and door locks will be part of what is resisting the twist, hence the doors becoming a little harder to open or shut.

When the hatch receiver liner is missing you'll hear a lot of scuttle as the tongue rattles side-to-side because of body flex moving the hatch.

Cabriolets are the pits in this regard: open both doors and look at it side on. The floor is the only thing holding it up in the middle. Floor has to be made very stiff hence the weight penalty of 40-50kgs. Ferrari say they have worked out a way for the doors to lock in to the body so they can do away with all the extra weight.

 

My '94 seems like it is carved from a piece of granite. The previous S4 models I've had did have more flex when lifted by one or two corners, however.