No - its not too late to join up and vote for Corbyn - there is still time.
I would suggest not putting 2nd or 3rd preferences on your ballot as they are going to try to use those to manipulate him out of the leadership.
"I suspect if he is winning it will be due to conservative politicians and supporters paying three quid and voting for their preferred candidate."
"Corbyn is a step left that appeals to CURRENT members but won't win new members."
I have never previously joined any political organisation.
I joined Labour (chose full membership price) last week only because of Corbyn.
I know 4 other people of various political position (but none being tory voters) who also rushed to join Labour in the last couple of weeks. People who have never previously been members of any political party and they have all joined just to vote for Corbyn. I have been amazed how he has brought out activity and enthusiasm in people who until a couple of weeks ago had given up hope with UK politics and its corruption.
People are desperate for democracy and genuine democracy needs to offer free choice. Labour, Libral and Conservative have all been right of centre parties for some while - so there is no genuine variation in choice for voters.
A good opposition is vital to promote debate, to highlight options and alternative beliefs or choices. Similarity of recent politics does not offer even a decent protest vote to the population.
Corbyn is the only person offering DEMOCRATIC CHOICES for voters in the UK at this point in time instead of just more of the same. Until the last week or so (and Corbyn standing) we have lived the last decade in what is alarmingly near a 'one party / one policy set' state due to all parties becoming so similar, with most politicians seeing politics as a good career rather than as a belief or as a need to help others instead of helping themselves to perks and power.
A good opposition is healthy challenge for whichever party (left, right or other) is in government and good for the population as a whole. Corbyn can reinstate an intelligent and engaged opposition - good for all of us, whatever our political party preferences are.