Def,
Like you, I "don't see how you can have T+0 settlement without the counter party delivering the stock on T+0." We are in agreement there. That is not the issue for me, rather it is: "CAN counter parties deliver the stock in T+0?"
One of Fohat's contribution's to this discussion is his point that SEC regs/rules are not a barrier to T+O. That is, in answering the question, "CAN counter parties deliver the stock in T+0?" we cannot say, "No, they can't, because it is not permitted by SEC regs/rules."
I have a similar response to your statement that "Funds are delivered to the clearing house when the clearing house demands them." The issue for me is "CAN the clearing house demand them earlier than T+3? CAN it demand them in T+0?" I haven't heard anything to suggest that they cannot demand them in T+0 other than the argument that T+3 is current industry standard.
IF the clearing house can demand T+0 fund delivery, then your scenario becomes: A buys stock from B and gets T+0 delivery. A then sells to C ...."
As yet, no one has raised the issue of the administrative and technological hassle to implement T+0 (I suspect it would be large), but that has not been the question on this tread. (People have raised the political issue of whether brokerages would favor the change.)
Thanks for saying you personally hope 0 day clearing is allowed. I agree with you that we smaller traders can use every break we can get.