You could make it that each god has a yin/yang aspect of duality - most folk promote/favor/worship the "good" form, and fear/appease the negative aspect hoping to keep it at bay, perhaps calling down the "bad" side on individuals they wish to curse. The "alternate" names are simply titles applied when these deities take on certain aspects.
I think you could roll Lolth into a "corrupted" aspect of Selune, or perhaps Shar, working quietly with the Drow to overthrow Lathandar/Corellian.
As someone else noted, Tymora, goddess of Luck seems like a good addition to the above - friend to Halflings, cohort to Gnomish pranks (as Garl) and secretly watching over the shoulder of rogues and gamblers (becoming Beshaba when she's annoyed or angered). Could roll Sune into Tymora's portfolio as the "fickle lover" who loves those who primp and preen.
Moradin could be a sterner aspect of Gond, as seen through Dwarven eyes.
Silvanus might be the Lord of Nature, with the aspects of Malar being the "wild hunt" form that seek vengeance against those who desecrate nature, Umberlee the disenchanted guardian of the sea and Auril the cold face of Silvanus's wintry wrath.
Cyric could be your "insane" god, with an evershifting aspect as Cyric (the lucid one), Tharizdun (utterly lost in insanity) and/or Moander (the liar and bender of truth, the rot and slow descent into madness).
<edit>Kelemvor's dark aspect might be Baal. While Kelemvor will patiently wait for you to die at your appointed time, Baal is impatient and wants you dead now. Good aspect for Assassins to call on, as well as for those who want others dead for one reason or another.
