I have never in my life seen a person fall asleep during an exam. You are right, she is talented - also in some dubious ways. Seriously, though, you must be correct: much of her bad traits are a direct result of Ui worshipping the ground under her feet and doing everything for her and Nodoka finishing the rest when Ui can't be present.

That the others also cover for her weaknesses is somewhat natural since their primary form of fellowship is the band and helping Yui is unfortunately the easiest way for making sure the band stays operational.

The preview makes me think Mugi's family must have a permanent holiday house in Finland because this makes the second time already during the series. I have to visit it for a cup of tea and some cakes one day...