Similar to I-190, 0 already exists as an option. Unfortunately, in this case, it's on the opposite end of the tollway. It was treated as running W->E, and so "0" is at the NORTH end, rather than S->N. Changing the south end to 0 can be done but would mess up list files.
I-55 is a hidden/alt point label at 23B since that's where it crosses the mainline. It's held over from when the Tollway exits were not numbered. Although SB 294 to 55 is not 23C in the field, I think it makes the most sense for the situation.