Most board bows have a riser on the belly side only.
your riser length should probably be somewhere between 5 and 10 inches- Your bow is plenty long,, so if the riser is "too long" creating a stiff section , you will still have plenty of wood bending.( This depends somewhat on drawlength- I'm assuming you're drawing to about 28.)
As far as cutting a "notch" for the arrow- I recomend against it. It can create a weak spot, and even if it doesn't, it's permanent. Instead, you could glue on a wedge of leather. this is moveable and is less risky for a beginner.
not sure what you mean by "Also wondering when I tiller down the bow do I keep it the same width all the way to the tips? (tips are 1/2") " . do you mean "thickness" where you say width?
Yes it's true that the tips do not bend- with your bow, you could make the tips stiff for anywhere from 2 to 4 inches. This kinda happens naturally, you won't have to make those stiff tips any thicker than the bendy portions. Both the width and thickness will taper towards the tips.
good luck!