So in your program what length of an arrow (say the Maxium Hunter dual spine weight forward 450's) should I be shooting?
I don't have the program on the computer here at the house but with your specs the 450's with a 100 grain tip and fletched with blazers you should cut the arrows to about 28 - 29 inches. If TFOX chimes in again I think he has OT2 on his computer. He may be able to run the numbers for you.
1- Does 1" of draw really cause this much change in spine? My cousin shoots 29" and has his arrows cut to 29" (same as mine) and he has zero issues?
Yes, it absolutely can make a huge difference.
Is your cousin shooting the exact same bow with the exact same draw weight?
But right off the bat his arrows are 1" shorter and his draw length is 1" shorter. Both of these contribute to needing a less stiff arrow. If he is shooting a different bow with a different cam this would also effect the arrow needed.
2- How come their charts and another software program (I had asked on this forum, I will try and dig up the advice I received a while back) show that I should be fine?
The charts are only a very general guide line. For instance...... take two bows both set to 70 pounds and 30" DL. One bow has a 6" brace height, dual cams and an IBO of 350 fps. The other bow has an 8" brace height, a soft single cam and an IBO of 320 fps. These two bows would need very drastically different arrows set ups but if you just go by the numbers on the charts you will end up with very wrong arrows for one or both of the bows.