What I meant was, which brunette are you talking about (cuz there's a few of them)? Triplet Maddy, office girl Brooke, waitress Vanessa? In any case, the only one you have actual sex with thus far is Vanessa. There's still situations with the others depending on your choices.
Depends on your definition of "best". I've done all her routes, but the full dark route with her is...extreme? I also am not a fan of subjecting people to unnecessary BS. IMHO, go either Lust or Purity route...with everyone. Or just pick whatever the hell you want. I have multiple playthroughs with full Purity, Lust, and Darkness; as well as a "mixed".
Zoey Raven does walkthroughs for many games on their Patreon, and has one for Radiant, though only to end of chapter 3. Check the master list. And it's free.