That David Miliband is the candidate of the New Labour machine, and committed to keeping Labour undemocracy as bad as it is or worse, with only the sop of an elected chair, is not speculative.
That Ed Miliband is campaigning on the basis of a "change" from New Labour vaguely in the direction of less undemocracy and some reconnection with the working class, is not speculative.
That Ed Miliband is the preferred candidate of the big unions, who are solidly on record for less undemocracy in the Labour Party and for a less sharply neoliberal policy (even though they have been ultra-weak in pushing those preferences), is not speculative.
That an Ed Miliband victory will tend to open things up a bit in the Labour Party, and a David Miliband victory to close them down, is - I think - consequently not speculative.
That Ed Miliband himself is, politically, no better than a weak "social liberal", is also not speculative.
The role Ed Miliband himself plays - whether he actively pushes the things which the unions and CLPs will be encouraged by his victory to push, or tries to fob them off, or (more likely) what combination of genuine concessions and fobbing-off he uses - that is speculative.
What sort or level of revival of rank and file labour movement assertiveness and stroppiness will be triggered by cuts battles, that also is speculative.
But the Ed Miliband 2 argument does not depend on the speculative things. It is based on the facts so far.