I have a Redbud planted in semi-shade. It does well and looks great with the nearby dogwoods.

Oakleaf hydrangea, perharps, though it need to be watered untill established.

I started to reply to this a few days ago but am not sure I'm on the same page as the OP. I think of an understory planting as meant to set off the trunks of tall trees, not fill in all the space between them, effectively obscuring that nice, somewhat open, woodsy feeling.

So, personally, I would not plant anything massive and lump-shaped like kerria or yew. I'd go for smaller, more open shapes, either low or with a few trunks, and ground-covers, like early spring bulbs, wild strawberry, or hellebores, around those. If your understory plants reach the height of your lower tree branches, and fill in that space, that's a totally different look, good for screening but not ... special. If that's what you want, then your plant list is fine, but if you want a real understory planting, that's something else completely.

You might add ilex glabra to the list, it's a native, non-spiny evergreen that likes shade and always looks good. Black berries on females if males are also planted. I've got blueberries in light shade, and they do really well there, too, although the birds get most of the berries. Vaccinium is s a pretty shrub, with great fall color, and comes in many sizes.