So many questions...

1. Bone is from cow-leg, by us known as "marrowbone". Fine soup can be made from it.
It has not totally circle shape cross section... it's bit triangular

. The longer, bigger and older animals are better...
2. Poundage will be about 25lbs(larp bows 25-30 by us). It can be adjusted by changing the string length. If 26 lbs... you need a bit longer string.
But this option is not endless, you heve to plan the prod-form before cutting it!!!
3. I have a 20lbs glass fiber bow from my childhood, so I think it can be easily pulled while placing the end on your stomack. Making stirrup takes no time compared to bone parts... it looks damm good too.
4. I have too ask the steel type. Extreme heating should be avioded by cutting the form...
5. I live in the EU(Middle-East-Europe), we got dozens of ruined castles to play there...