I don't know how you load your local files, but they open just fine if I long-press them in FX file explorer and select opera.
I tested with files on my phone, and on my micro SD.
and I don't think I've ever seen that rate popup, at least not more than once.
as for getting back to opera mobile classic:
try "updating" opera 18 with this:
http://ftp.opera.com/pub/opera/android/mobile/1200/
as long as you haven't messed too much with the opera 18 settings and speed dial it should work fine.
then activate opera link to backup all your bookmarks to an online account.
after that install opera mobile classic from the play store, and login to link to get all your bookmarks back without having to worry about updates.
after that you can decide for yourself if you want to update to 18 to test future updates, or delete 12.00 and stay with classic permanently.