luthien (i have long thought) is the real 'star' and hero of the lay of leithian, much moreso than beren. it's very apparent through all available materials on the subject that luthien was to a large extent intended as a stand-in - in the elevated, mythic context of tolkien's world of elves and gods - for his own wife edith, he 'holds the character up' as he likely did his own wife (as many people do) and seems to have no difficulty painting beren - being similarly a version of himself - as second fiddle in their tale. to that end yes, the character, though accounted as one of the legendary heroes and elf-friends of the elder days, seems more an 'ordinary man' - and in my vision probably less tall and naturally regal, less 'elf like' you might say - than turin or tuor. there's a little bit of bilbo (tolkien's other not-so-subtle literary avatar) or sam gamgee in beren - dogged, out of his depth, an honorable thief, his main advantages being his courage and good luck - and it seems to me that among the great heroes of the tales of the first age ("the great stories,' as sam himself memorably intones) he would be the most relateable to those later figures.
concerning the composition (because you remarked on it and because it's been so far my most challenging one, trying to get the scale, energy, and motion of the characters all right in themselves and in relation to eachother, frozen in a single moment of tension) my hope (which i'm hoping will be fully realized when the image is finally rendered up to a resolution I can consider myself happy with) is that the character dynamics of the scene will take center stage: Celegorm and Beren, and their interplay, are intended to be the most dire element (that spear will likely fly in another fraction of a second) with celegorm, the emotionally "hottest" character of the four, driving in on beren, the barreling mass of his horse forcing the lovers apart as he directs all his rage and jealousy at the man under him (i consider it a moment closely akin to johnny's beating of daniel in 'karate kid,' cocoum's attack on john smith in 'pocahontas,' cal's attempt to shoot jack in 'titanic' or any one of the other (literally dozens of) iterations of the boy-from-the-other-side-of-the-tracks-wins-the-girl's-heart-and-incurrs-the-wrath-of-male-competitors-from-her-world story (one of the most common and reliable of romantic archetypes)). Luthien and curufin (who is essentially there to help his bro get some) are second-stringers, with luthien's attention just shifting to the darker noldor prince's approach, but my hope (and it sounds like you think I'm succeeding) is that, by placing luthien in the center of the scene, as the only character "engaging" all the others (one might expect she would cry a warning to beren, or to celegorm to stop, or to curufin to stop him, if the moment allowed her enough syllables) and as the "object" of all the other's focus (either directly or indirectly) she will read as the figure around whom all the action of the scene pivots.
my apologies if that got long-winded
but, as this is a WIP i am still somewhat pre-gaming it myself, and often i find that doing so in writing is effective. hopefully wasnt too much of a burden to you