I've failed at getting all the hits off the second 3c multiple times before, but never really looked into why.
That said, I don't see any reason why your 23c6a and 3c236a should position you any differently, unless you're cutting your dash short by crouching earlier in the first variation.
Fairly unrelated, but does anyone else have issues landing the j.2c at the end of air combos? I tend to knock the other player too high (or something), so I'm wondering if it's just a height issue, or if my timing is off in some form or another. I've gotten fairly decent at being able to tell if I can pull off the j.2c, but it seems like I'm just doing something wrong if I'm forced to do the j.b j.c j.236a finish most of the time.
Random note: for 5cdc 236b dash 22a, I've found that inputting 6632 for the dash, then a 2a works for me most of the time. Takes a bit of effort to remember to input the 2a, though.