Amongst the more conspiratorily minded there are two themes:The...

Amongst the more conspiratorily minded there are two themes:
The early debate was to give the Democratic party schemers room to manoeuvre before the August Democratic Party convention.
Plausible, but I think the real reason was slightly different: Biden's handlers know how fast he's declining, and wanted to get one good debate in this early, before it became unachievable in September/October. Turns out Biden's gone downhill further and faster than they were counting on, so yes: they were given a nasty surprise.

Michelle Obama as a 11th hour switch-in,
Implausible. You seem to forget a substantial criticism of Hilary Clinton's nomination was that she would be a wrapper for Pseudo-President Bill Clinton to return to the White House and run the show off-camera. In Hilary's case she had personal political prominence before and especially after the Clinton presidency, as Obama's Secretary of State.
Michelle Obama has not held elected or appointed political office. There is no way she will parachuted into the Presidential campaign.
(Although I thought the 2nd Coming of Kevin Rudd was impossible, and look how that turned out ...)

Meanwhile, the Democrats forcing themselves to believe Biden is viable is truly astonishing. He's barely likely to be alive in November.

Dark and terrible times for the USA.