Context is everything.
In GameLogic.opt there are parameters influencing XP gain:
'expXPGainWhenHittingH = 50 // this value of hundredth of xp when hitting an enemy'
So 50 hundredths or 50/100 = 1/2 = 0.5 -- so for every hit scored by an element, the element scoring the hit gains 0.5 XP.
'expXpGainCoeffLeaderKill = 10 // Leaders get 10% of their subordinate SU xp gain'
SU=SubUnit (less than a unit) = elements - in this context, those elements commanded by a leader)
So 'expXpGainCoeffLeaderKill' pertains to the leader directly in command of an element. A division commander is directly in command of all of the elements inside his division. If you combine a leader with a brigade, the situation would be identical, but with fewer elements under the command of the leader. But Corps Commanders also gain XP in combat, although they may have no elements directly under their command, if they only have divisions inside their Corps Stack.
IIRC I read in the forum somewhere that Corps Commanders gain 5% of the XP gained by their SU's, and Army Commanders 3%. I believe the 5% and 3% are hard-coded into the game engine, which is why there are no parameters in the GameLogic.opt file.
I have never heard of loose leaders in a stack being assigned command over loose units. I cannot 100% refute the possibility, but I've simply never heard of it.
I do have a very vivid recollection of having some of those special engineer units the Union gets later in the war (Railway Bde, Merchant's Bde, etc.) getting mauled in very hard battles when they were loose inside a Corps Stack, even if there were loose leaders in the same Corps Stack. I discovered through Trial-n-Error™ if I combined these special engineer units with a loose leader in the same Corps Stack, their survivability increased greatly. So, from my experience, if loose leaders were somehow assigned command over loose units in the same stack, I would not have discovered to combine these engineer units with a leader, to prevent them from being destroyed.
I've also never heard that loose leaders in a stack providing an ability (Cav Leader, Artilerist, etc.) are considered to command elements of the type they modify, but it would be logical to some degree, but I have no actual knowledge of it.