or JD's 4 color Belcher
I think you meant 3c.

I was not aware that I had a 4 color Belcher.

Abeyance can be cast off a land and any of an extra Ritual mana, Mox Ruby/Emerald/Pearl, Sol Ring, Mana Crypt, or Mana Vault. Basically, you're only going to have to tap one land (well, bust one 5c source), which is key. Blue mana is the hardest to give up in 5c combo decks, since you can't easily make more of it (black mana makes more black mana, an important thing to consider), and you always want to keep a blue mana around as long as you possibly can. Your lands are the best source of blue mana you have, unless you've been fortunate enough to locate Lotus or Sapphire. If you tutor for Lotus, you usually had to give up BB to do it, and you'll often need to crack Lotus for black to make up for that. I think I end up craking Lotus for BBB about 75% of the time, compared to the 25% of the time it goes for blue. It goes for blue usually only when you have Ancestral Recall AND Brainstorm in hand (or a MT instead of one of those), or Desire, or you have two rituals (and therefore won't be hurting for black). You really don't want to have to tap two lands to cast something if you can avoid it, since you want to win on turn 2-3 and need at least one land for black to cast Ritual, one land for an additional blue for Brainstorm/Tinker/Recall/Chain of Vapor (for storm), etc. That either runs you out of land or leaves you with only one left to tap for both blue and green...
Thus, Determined is harder to cast than Abeyance, and the limiting factor in the casting cost is not the green mana, it's the blue. If you were casting Abeyance, you would need to tap a land anyway, and that land could go for green instead of white.
Xantid Swarm is still the king when it comes to stopping countermagic. At one mana, it walks into Mana Drain all day without worries. Essentially, it's like a Duress on steroids, as it either two for 1's (they Force it), counts as two spells for storm (they Drain it), does both those things (they Force it), or resolves and you win on your next turn.