Once you've unlocked all kingdoms (that first!) you could also try to start working towards traiting Khorvash. Those stones drop at Broken Spire or Blackhawk. All his traits are worthwhile.
About the team bonuses, the lower your average stats are (the lower your level) the more useful they are, but they only help to offset a certain amount of missed synergy. The team you are building is already more synergetic than you can hope to compensate with team bonuses.
As a reference, I am currently (mid to endgame) PvPing with a team of Khorvash, Queen Mab, Valkyrie and Famine (last slot replacable by Mercy or Snowman too) (Double yellow banner if with Famine, Dragon otherwise), and still going strong with that.
My quick soulfarming/traitstone farming team is Queen Mab/Rowanne/Valkyrie/Mercy (Dragon Banner) where the three traits on Mercy are the most important.