I'm not sure if anythings drastically changed in the last few months since I dropped Tohno but I don't really see a "major" problem with whiffing 2a besides the fact that your creating an unnecessary hassle for yourself. As you're already in neutral anyways from the 5bb, whiffing the 5a supposedly makes the timing/jump easier. I have to ask though, I don't see why it wouldn't be frame wise but is that combo valid?
Also, your listing of the air combos is off as it's impossible to j-cb then dj-cbc without it being an invalid combo (or in general, who knows I'm to lazy to test it).
In general here is what IIRC is possible from the whiff cancel combo:
j-c dj-bc (for those to lazy or apprehensive to go for the b in the first jump)
j-ca dj-bc (this is what you can use if you think you miss-timed the delay on the launch)
http://www.youtube.com/watch?v=DTi33Ze6H3Mj-cb dj-bc (the most commonly used combo)
http://www.youtube.com/watch?v=asBkK2UC26cand IIRC there is a j-cab(or ba) dj-abc(cba?) (something like that, but it's hard to do and not actually worth the minuscule amount of extra dmg you get from it)
Generally once you get in the air you are stuck at (I believe)
40% 55% proration so no matter what you throw out (within reason) you won't prorate any further, so if you can throw a j-cab dj-abc (or whatever it is) and land it, might as well go for it.