Black is definately the Way To Go(tm). The big thing to beat with this style of deck is countermagic, meaning discard and control elements are very important. Your Entombs and BAs will put the stuff you need in your graveyard without too much trouble, but I'd want to focus more on the combo and protecting it than on control. Playing counter-for-counter in combo decks is pretty bad unless you can do dumb things like Thwart and FoW, so I think it might be better to invest in discard effects to prevent your opponent's counters from ever going through. 4x Duress is obviously the first choice.
Oddly, I'm voting Mesmeric Fiend for #2, thanks to his ability to play ultra-Duress on turn 2. His only real weakness is that he doesn't help you acquire Threshold - If you're worried about getting the Rituals to work at ALL, it might be better to run Addle, which serves largely the same purpose.
On the side of "Here's a wierd suggestion," have you considered running one or two Soul Scourges (Nightmare for 3 life)? They're an odd choice, but you don't get the ability to insta-kill via the Call unless you have all four Champs in your graveyard, meaning drawing even one puts you in some trouble. Three Champs and a Scourge, though, is a kill in and of itself, leaving you a few more options for the instant kill attack.
Another option along the same lines of making sure you don't lose if you draw a Champ is to run Insidious Dreams over the Tutor. I don't think it's NEARLY as good, but it is an option.
Finally, it might be better to run Sleight of Hand, Opt, or similar quick search cards over any remaining counter effects. They'll get you to Threshold unfairly fast, and they're cheap search engines. Worth a shot?