Hi Owen,
The good news is that both these requests will be met by some code I'm just finishing up and will be in the next major release (4.1.0). I can't promise when that will be released, but it will be feature packed.
Essentially you will be able to change/delete any single instance of a recurring event, without impacting all the other instances.
In the current world, I would guess that your change of the date/time is having an impact on the recurrence logic. Without looking at it in detail, I can't be 100%. But if you are doing an update, there is no way you can delete the event. Bear in mind that currently all occurences have only one record in the database, so when you edit an event, you are in fact always editing all of them. This is the major change to the 4.1.0 version, where each occurence of an event has it's own record in the database, so can be manipulated independantly.
Cheers
Roger