Thanks for the links -- I especially enjoyed the reading the first, which gave me a better working knowledge of the technicalities behind the workarounds I've already been employing.
I've finally got Opera/Gmail working together in a manner that, while imperfect and a bit limited, I'm generally satisfied with.
Mostly it's adjusting how I interact with it, not so much actual configuration. I think what I'm doing now more or less accomodates gmail's quirks.
1) On the server, I removed all non-system labels. Opera's inline search is fast enough to negate the need for them anyway
2) Generally, I always work with the Imap folders, rather than Opera's 'unread'/'received mail' to avoid confusion. For example, to maintain that I still have an inbox when I use the web interface of gmail, I don't auto-archive everything. Subsequently, every new message arrives twice--one for all mail, one for the inbox. So I read the Inbox folder for new mail.
3) When I'm done with a message in the inbox, I mark it as read and delete it. The all mail copy still exists and will also be marked read (though opera is sometimes a bit slow to notice it's read; probably because of the size of my mailbase)
4) I never delete an item I want to get rid of, always drag to spam or trash from the inbox/all mail
Following those rules, Opera and GMail are getting along pretty well for the most part. Assuming there aren't horrible things happening in the background that I"m unaware of. (Lost items, duplicates, who knows?)