I don’t have specific knowlege of your car but I do believe that the most commonly overlooked engine maintainence is valve clearences.Find a mechanic that has the shims and have them set on the loose side of specifications.
“Would the change in fuel mixture show as a gradual chance in mpg on an instantaneous (per fillup) mpg chart, or would it be more of a sudden change with a standard range for winter and summer?”
Yes …
Generally it will change over a matter of weeks. The tank at the station will not be empty when the new fuel is added so it becomes a blend. Even the fuel being delivered can be a blend from the tank farm. So it does change, but it should not take too long to be mostly changed out, a few weeks likely. It is also possible that they may intentionally have an intermediate mix, but I don’t know if they do that anywhere.