You’re right, @ScruffR… you need LEDCOUNT/2 + 1. It has been too long since i reviewed that site. But as for the end frame, the fact is the two chips are not 1:1 compatible. “Ones” work for the APA102 but not for the SK9822, and the site you linked suggests using ones is acceptable. This is why FastLED added a new class for the SK9822.
I have not seen the effect of non-set frames going all white, but perhaps that’s because I’ve always relied on libraries.
@kasper I’ll ask my friend to chime in here about GBC.