Thanks for the interesting analysis.
The problem at the moment seems untractable, but it's early days yet.
A red/green minority govt. under Schröder might get elected in parliament in a secret ballot, even although the Left party leaders have said they will NOT support Schröder even then, let alone join a coalition (not that anybody would have them). But secret means secret, so nobody will ever know if some of them (or even FDP and CDU MPs!) vote for Schröder.
But a minority govt. would not be able to get much past the CDU-dominated Bundesrat.
On the other hand, nor would a minority CDU/FDP govt. get anything really radical past the Bundestag without the support of one or more of the SPD or Greens or Left party. So all bills would have to be based on general consensus, preferably with the SPD for a really solid backing.
So both governments would be very vulnerable.
Schröder should be careful not to overplay his hand, or he'll forfeit sympathy. Even left-wing voters are starting to feel rather sorry for hapless Angela Merkel...