What's the safest shape for homemade hard-candy
Spherical or spheroidal foods are indeed the most likely to be implicated in choking - grapes are well known. But you're unlikely to get sufficient statistics to answer fully. Most of the medical literature I've seen is at least as concerned with size, and of course that changes as you eat the sweet. You'd have to start unusually small for size to be much help. 

Hard edges are much less of a concern. You'd struggle to get them sharp enough to do any actual damage, and the corners dissolve fastest, meaning they blunt almost immediately. This also means that the corners of something like a cube will go fairly fast, and you'll be left with a near-spherical blob.

You might be best off with something like a coin shape - a flattish disc or oval. This is a traditional shape for barley sugars, and I've also had hard fruit sweets that shape. While coins do show up in the choking statistics, they're very common items for children to investigate, including by taste and when not supervised.