Thanks for splitting up the patch. IMO it could have been split up further, with one patch addressing the comment capitalisations (which would be accepted immediately) and the other with the code refactoring. For now it's fine to keep as-is.

I think it's still a good idea to split up the init function into three parts, for example like in the patch below:

• This makes it possible to directly return certain structs, rather than rely on return parameters. This in turn makes in possible to declare rectf as const, improving understandability of the rest of the code.
• It makes it clearer where that pesky void *data is going to; often such a parameter is passed to ANIM_animdata_filter() instead of ac->data, and I expected the same to happen here.
• It also showed that the r_filter return parameter wasn't even necessary.

I didn't put much thought into the function names, though -- I just used "name of the original init fuction" + "some word that somewhat describes this function". I'm fairly sure better naming is possible.

If you want to clean up the code even more, refactor initialize_box_select_key_editing_data() to use a switch() instead of a sequence of if/else if/else, and replace r_ked->data = data; with r_ked->data = data_circle to make it consistent with the case above it. For me that's just a nice readability improvement, but not a requirement for the acceptance of this patch.