In my opinion, the same keys could be used for adjusting the bipod legs as those for adjusting the turret height (which are also used for adjusting the periscope height on the Strider, for example). In the View category of the Controls configuration, scroll to the bottom and see "Raise turret" and "Lower turret".

The US Army field manual FM 3-50(90) has charts of electro-optical systems and the types of obscurants that defeat them.

The improvements to the DayZ animation system with "proper turning animations" seem very promising for accomplishing a fix to this issue in Arma 3.

VBS2 has had "ground cutting" for years. See this demo video. So this is possible on a technical level.

Either that or the positioning of the player is different in the internal build. Is the player's bipod on a slope that declines toward the enemy position and the player's body on a slope that declines away from the enemy position? Maybe you could record a video of how it looks in the internal build?

Here is a video demonstrating the problem. I took my hand off the mouse (so that I didn't accidentally change the direction that the player was facing), pressed the key to go prone, pressed the key to deploy the bipod and then held Alternate to look at different angles. Clearly the player is hovering in the air, supported only by the bipod.

As of version 1.80.143869 the problem appears to be resolved.