de di,
As you noticed, the start-index mechanism does not work for
synchronization purposes because there is no way to determine if you
missed a comment because one of the comments was removed from the list
for some reason while you were fetching.
Unfortunately we don't really have a dedicated system for exporting
comments right now. The API endpoint you mentioned is designed to drive
a comment management interface rather than a synchronization capability.
My best suggestion is to do an export to MTIF, a simple text-based
format for representing blog content, and then consume this result. This
won't allow you to consume new comments without doing a new export, of
course. MTIF is documented here:
http://www.sixapart.com/movabletype/docs/mtimport.html
I'd be interested to hear more details about what you're trying to do;
we may be able to adjust the API in future to provide something closer
to what you need.
Thanks.
Martin