In answer to the questions raised:
Coming from the South West, change at Clapham, head for Platform 13! IIRC and there are lots of trains to East Croydon. You are unlikely to have to wait more than 10 mins tops and the fast trains take about 10 minutes to get to EC. The ride although short, is a bit horrid and not recommended unless you know the route.
Joining on route, shouldn't be a problem. I didn't realise there were helmet rules, I don't wear one usually but can put a motorcycle helmet on if needed.
Knowing where to go - I used to be a delivery driver and have worked around town for the last 20 years, the route we will be following is one I have travelled many times before (almost daily during the Olympics). It is always a pleasure to share and I'm sure the RetroBikers will find new twists and turns that I have missed.
London is a 24 hour city, the optional bit on the return via Crystal Palace is actually much better at dusk or night. I was anticipating a 10am departure.
The consensus on yesterday's ride was that due to ever shortening days and other events, it would be good to run this route in Spring, early March maybe?