Targets3D, Don't look at the change from Oly to Trad as a different process. The form remains the same. You may go to a side of the face anchor rather than under the chin but the only thing that really changes is the gap you would use. My experience is that if done correctly you will find little to no change in draw length.
As far as the bow arm is concerned, don't try to separate your two techniques you mention above. Keep your arm muscles relaxed, bow shoulder down and back and as you come to full draw, press your bow shoulder and bow hand toward the target. This is part of the "expansion" step, the other part is the final drawing arm motion leading to release.
The Olympic BEST (Biomechanically Efficient Shooting Technique) system works equally well for formal target shooting and what we consider traditional style target,3D, and hunting. The real difference in using it for Trad shooting is that we adapt to the shooting situation and are less "by the numbers" or said another way we're more casual in our shooting.
I'd suggest that you continue what you know and work on shooting from less than "flat" ground, you will discover a technique that fits you the best but keep the form you already apparently have from the formal target archery arena.
Good luck.