Your a brave man taking that photo with the 2*4 between your legs. If one of those limbs gave way that board could act like an arrow and try to fly but since it can't go forward or down there is only one option
Any spot that has less wood than its corresponding spot, i.e. that thin spot, will be weaker. The bow will bend more just in that spot causing a hinge.
Hinges are bad
You do not need to cut the arrow shelf to center. In reality you do not need any shelf at all. If your handle is wide then you just use a weaker spined arrow and it will bend around the handle, while still shooting straight. Check out the thread by 4est Trekker The "So You Wanna Build a Bow?" Build-Along in the how to forum. Great resources for building a pyramid bow.
http://tradgang.com/noncgi/ultimatebb.php?ubb=get_topic;f=2;t=000122