Don't have an opinion on the willow, but i do on the oak. I suspect that a lot of your cast and set problem (actually, all) is due to three factors:
1. too narrow (oak needs to be at least 2" wide for a decent weight),
and yes a pyramid is one of the best designs for oak2. crowned as opposed to flat belly
(these two have been identified, but there's a third...)
3. a garage kept and tillered bow during the rainy months. I'd say that raised your mc and caused a lot of cast-robbing set.
If you don't live in a dry-ish climate, temper the belly (either way it will increase cast). As far as splinters, i'd also suspect the board selection based on your statments ("all edges slightly off parallel"). You also have to watch the actual grain direction in oak (any board for that matter), not just the ring orientation.
I'd say try a great tempered design on a prime red oak stave, and you'll probably give up on trying willow