Frames Question

I have some questions about frames.

1.If a low grab takes 8 frames execution and you block a low d+p there at a -4 disadvantage and if they try to do it again it would take 16 frames execution to come out so why can't the low grab grab them after you block the d+1?

 

because you can't interrupt an attack with a low throw, the game doesn't allow it. In general you can't interrupt attacks with throws, except for special catch throws.
However, some may be exceptions... like possibly vanessa's low throw.
The tradeoff is that a low catch throw won't come out as fast (won't be 8 frames).