I feel your pain! I'm a full time computer programmer and it took many sleepless nights to import the 650 events for our stake calendar (half of which were all day events, improperly imported).
I was able to import everything, but half of it showed up at the wrong time, the wrong day, or wasn't properly recognized as multi-day. I did it a month at a time (importing a month in .ics format, then manually editing the events), and March would behave differently than May, because of daylight savings. The same applied at the end of the year, in reverse.
In the end I'm left with the same conclusion: An unpredictable, unreliable import was worse than no import.
A year ago when there was no import I wrote a command-line program on my server that would log in, submit a single event, wait 5 seconds, then do it again for the next event. It worked great, was reliable, and took very little of my time (and no additional server load than doing it manually, same number of page loads, spaced out). I wrote and ran the program in easily 1/4 the time that it took me to do the import this year, and without any of the stress. Unfortunately I wasn't able to get the same technique to work with the Web 2.0 approach this year.
The iCal "standard" is unpredictable and unreliable in itself. The great thing about iCal standards is there are so many implementations to choose from! So I can see why the programmers at CHQ may be feeling the same frustrations we are.
My suggestion: Have a separate import using a CHQ-defined CSV format. It should take next to nothing to create (and I'm not just saying that, I create CSV to database imports regularly as part of my job), would be completely reliable, and ~most~ wards / stakes have someone that could generate the file. A field for the time, date, title, description, and a 0 or 1 to specify whether it was an all day event. Then have a dropdown to choose the timezone when you import the file. Done.
A simple, reliable CSV import would have saved me easily 12 - 15 very frustrating hours.
That being said, I really like the new calendar and directory, am plenty happy to see the old one go away, and commend those working on it.
But I really, really hope this is fixed before next year. I'd hate to have to move away from a stake I love just to avoid what should be a simple calendar import. But the heart-attack / stroke risk of a repeat isn't fair to my family... They need their Dad.
