No not once the subscription expires all access to idisk and email and other features stop. I am aware there was at least when I renewed my account late last year a 14 day period before they delete the account and any info.
I have been a mac user for around 10 years and have never seen one sudden reboot for no apparent reason.
I have seen two kernal panics but they were easy fixed.
The type of mac would help try and figure out what is happening.
The download size looks like it will be 773 MB but don't forget the install size is more likely bigger than 1GB.
So far I have only seen a issue using USB flash drives acting funny and sometimes just disappearing off the screen without being touched.
Can you please post some more info about how your imac is crashing.
If it crashes with certain apps or system related crashes.
This will help give a better answer to what could be the cause of the crashes you see.
Try repairing the permissions as flash player screws quite a few up when installed. I have noticed after they have been repaired things seem to work much better.
I have also seen similar problems under windows with youtube and flash crashes running the latest stable version.
I mainly run macs but have a pc for my work related stuff and always do some tests with it to see if anything different happens.
Downloaded the combo updater and installed without issue.
Seems much snappier now and now all I need to do is buy a new mouse as the one I have is dying slowly.
I have noticed one thing has disappeared when repairing the permissions the
SUID regard private framworks and locum has now gone.
I will do some more testing of things later and post any issues I may find.