to answer some questions
QNH, or "altimeter setting" is what is used by the military in states. Is there a plan to change the voice comms in DCS to match this? it does affect terrain clearance, level deconfliction from other traffic and altitudes to climb to when trying to hook up with the tanker. Unless ofcourse everyone else is flying off the QFE at YOUR departure point. (unlikely)
The altitude you set QNE/ standard/ 29.92 inHg (or whatever you wish to call the transition altitude/level) is mainly for terrain considerations, and sometimes airspace. Spot heights on the charts are based on QNH which is the local altimeter setting corrected to sea level. So it is safe to say that you would need that to know where you are in elevation in relation to the terrain, and whether or not you are above MSA (minimum safe altitude), MEA, MORA, MOCA or whatever is depicted on your chart. Although at times some of the above are given in flight levels. The reason why the transition altitude is 18,000 in the states has somewhat due to the terrain in the mid west. They would rather have a uniform transition for the entire continent vs having one change from region to region, re: europe!
QNH in millibars or hPa, not a problem on jets as the altimeter scale has both hPa and in Hg.
as for us going to moscow or somewhere in china, we have tables for feet to metric conversion (one for China/North Korea, and one for Russia/Mongolia, as their RVSM levels differ somewhat) but for SIDs (or DP's) and STARS there is a metric to feet conversion on the Jeppessen plates. However we do not base our descent and approach on QFE, we request QNH from ATC.
QFE vs radar altimeter: QFE is referenced to a point on the airfield, and therefore indicates your elevation above (or below) that reference only. The radar altimeter however will bounce up and down depending on what is below you. including buildings, trees, birds, other planes or whatever solid strays under the radalt transmitter! You would lose all sense or where the airfield was in relation to you, after you do some maneuvering.
hope that answers some questions