Obviously the first thing coming to mind when hearing "murdered in a sex ritual" a drow comes to mind, so Minthara - she doesn't sound all that good :P Wouldn't also put it past Wyll (parhaps not willigly, but you know...he has a patron) or Astarion.
Astarion also comes to mind with the trurning into a vampire thing, but also... Shadowheart? Vampires are not really Shar's domain (?), but wouldn't put it past her to make everything uphill for her followers when they have too much of a good time, or, you know, fall in love.
And as for the last question... well, Karlach seems like a wife material from what we have seen. Also Gale is shown as a romantic (come on, that boat ride was at least swoon-worthy...unless that is the start of the sex ritual
).
Preferably all of them can have a happily ever-after though.
