Okay so I fiddle about with the 2B 5C 5B 2B situation in training for a bit today.
I find it noticeably easier to get the 5B 2B if I input a forward dash before the 2B 5C.
For example
FC 2C > 5B > 2B > 5C > 5B > 2B instead do FC 2C > 66 > 5B > 2B > 5C > 5B > 2B
It really feels like the timeframe to input the 5B is more forgiving when you dash forward a tiny bit first, obviously the opponent will be closer to you after the 5C doing this.
Obviously if you're starting on CH 5C you're pretty much at the mercy of wherever you were when you confirmed. But FC 2C gives you enough time to input a dash regardless.
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Similarly, it's much easier to land 236B~j214B on this:
...5C > 236B > 2A > 5B > 2B > 236B~j214B
by simply adding a microdash before 2A thus
...5C > 236B> 66.2A > 5B > 2B > 236B~j214B
Also, random observation:
Actually does 3993.
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Another random note:
CH 5A > 5C does combo successfully on standing. So you can do CH 5A > 5C > 2B > 236B~j214B > 236A/B