Hi there Timm,
Thanks for posting!
Just so we are on the same page - let me try to summarize here to make sure ive got it. You're trying to update details (Amount / address etc...) on an Opportunity based on details contained within the Most recent Estimate attached to this Opportunity, Correct?
Part 1 should be fairly straight forward. Updating the Opportunity from the estimate would be as simple as throwing in a button (or adding the actions to the exsting Save button) on screen which updates the selected Fields in your opportunity, based on a related field on the Estimate, where the Opportunity RecordID = The RecordID of the opportunity dropdown on the Estimate screen. Something like the screenshot below (Taken from the Estimate screen);
Let me know if this was the kind of thing you were looking to do, or If i've missed the mark here.
Thank Timm!
-Ben