For Ayane juggles off BT 2P launch (same attack as 6PP) use PP6K PP6PP. Ayane is best used as a whiff punisher. Be defensive and let your opponent come in. It helps that Jann Lee is heavy on his 66K so use her mid hold when you can and react to his 6PK.
I also think holds that are 20 frames total would be too fast. Most moves have more than 20 frames of recovery, which would mean you in most cases could instantly hold out of stun without fear of any repercussion and possible even be at an advantage.
Thing is, you can only get high-counter throws during the active frames of holds, so reducing the active frames that much more or less makes it impossible to throw-punish on reaction.
As for changing the entire game by altering holds and stuns like that, I don't think I know DOA3 well enough to properly judge what would be balanced and what wouldn't.