Where is the kidnapped Selunite option? Because I'm pretty sure, that's what she turns out to be.
That would be the most cringe way to go about it. Please don't do this whole light/darkness duality nonsense, it's so boring, just let her be this totally evil woman who may or may not be redeemed down the line.
It dont seem to me like theese two things are mutualy exclusive ...
I mean ...
Even if Shadowheart turns out to be brainwashed Selunite, she still spend quite some time in service of Shar ... and with our conciderable help, she may do some unspeakable horrors during that time ... also Selune never helped her, there is many stories about people who lost their faith for less.

I would never call it boring, or cringe ... as long as there will be option to pick.
On the contrary!
This story of someone who lived their whole life in service of good deity, but then were kidnapped, brainwashed ... and basicaly forced to commit atrocities in the name of evil deity ... only to discover their path later on, and now be forced to decide if they will return to the light, or refuse it for good ... sounds very interesting. ^_^
I loved it with Revan and would love it again.
