We got a crazy situation here. At least I find myself in a crazy situation
From what you said and from what i intended to do i only got these possible solutions left to chose from:
a. create a second player char to romance all npc (including both mod npc Fade and Amber)
Impossible for me cause i won't waste that party slot
Nope. you could create a second pc to romance Amber and romance Fade with the bhaalspawn pc. But if you don't want to waste the party slot, that's your call.
b. take both Amber and Fade and decide which romance to follow and which to end
Not very satisfying cause i won't find out which one has the better ending (okay okay you're right. I
won't find out this way nor the other)
You could peek at the epilogues in the mod folder if you felt really strongly about it, but that would be cheating. Never mind. You said you're ok with that.
c. take Amber or Fade
Hmmmmm. This means spending houndreds of hours (!) to play the game twice just to figure out which
one attracts me more
or start 2 games and pull the plug on one of them when you decide on your preference. AFAIK, neither has
ToB, tho.
d. take none of them
Not what i had in mind. I still want to enhance the already known game with some new female npc (i
never installed a modded npc before. Never liked them......)
I suppose... Hardly seems satisfying.
Guess i'll do some research to find out which girl fits me most. Then i'll take her and play the game using Jaheira and Vicci. Maybe i'll find out very soon that she isn't worth hanging on to and dump her (sorry). This way i can still keep on romancing Vicci and Jaheira 
By the time you do the research, you could have probably formed your own opinion by playing. But your call.
Fortunately, NPCs, mod or otherwise, are nothing but ones and zeros. They don't care if you dump them.