@engineer,
I suspect we'll see either Biden or a new Democrat candidate if Hillary continues to fall. The stakes are high for them, and I doubt they'll take the chance with Sanders. The problem is that introducing a new candidate (whether Biden or someone else) will add to Hillary's troubles at a critical moment... something they're reluctant to do precipitously. This is likely a very tough call for them, and as it is their last gasp, something the Clintons will let go of only when all hope is lost.
I hope Trump fades, but don't agree with you about Carson. In view of his lack of political experience Carson may well make a better candidate for VP, but I believe he is a serious person with a refreshing perspective that is engaging people now. He should be taken, more seriously than Trump.