Ok, I adapted my spreadsheet to process ics files in the same way that it processes vcf files. That is, it reads each calendar entry from the ics file into a row of the spreadsheet, with the date of the entry duplicated in column 1 to allow easy sorting. If I sort by date and cut out the past and future entries I'm not concerned about, I end up with just the next 4 months worth of calendar entries, re-export those to an ics file, and those import successfully to GPE Calendar.

I won't bother tracking down the "invalid character" problem or anything else unless it turns up in the current calendar entries that I'm trying to transfer. In that case I should be able to identify it easily enough and add a correction routine to my spreadsheet.

If anyone wants the ics calendar file version of the spreadsheet, just ask (update: attached).

Incidentally you can find the current calendar file in the same directory as contacts if you want to delete it and start fresh. Open a command line window with XTerm and type:

$ cd ~/.gpe
$ rm calendar

So in the end I am now able to transfer both my Contacts and my current Calendar from Yahoo online to GPE Contacts and Calendar on the N800 fairly easily, and I don't mind updating manually once in a while instead of trying to manage a complicated sync operation. I count that as a success.