Unfortunately this is still an open problem. Moving events from site to site, was the aim of the CSV extension, but this is still a working progress.
WordPress' XML export doesn't include event dates, however, it does at least import all the event's scheduling details from which the dates can be generated (as you noticed, after updating the event, the events appear).
iCal is the 'universal' way of communicating event data, however it is very limited in the sort of data it continues (so no SEO keywords for example).
In your case I would recommend using XML and then writing a script to run through events and update them.
I will look again at adding support for XML import for 3.0.0 (currently in development), but last a looked it wasn't possible to ensure that event dates were created when imported automatically. 3.0.0 shall be introducing WP Cli support, so it would be interesting to see what can be done there too.