You never fail to amaze me Chek! :mrgreen:
Though, I still have some problems with your deduction but only a fraction of it.
THE WIND: You claim that the wind was blowing towards the west, and slightly south. But I believe that the wind was headed in a northwest direction and I have a few reasons as to why.
In the picture above, Ran and Kogoro are walking with the umbrella facing the south; thus, the wind is blowing AGAINST them (heading north, not south). You can also consider their facial expressions and body positions. When walking against stormy winds, your face would tense up and make a "semi-frown-like" expression in order to resist the powerful breeze. You would also lean your body forward in order to prevent yourself from falling backwards. As for the flag, maybe the wind was blowing so strong that when it settled down a bit, the flag slightly whipped back to the south, then resumed waving NW. It can also explain how the umbrella hooked onto the car at the end of the parking lot. While Hatsune was being attacked, the umbrella was release from her hands and departed northwest towards the other car.
THE FOOTPRINTS: I believe that walk-rolling on the umbrella is too far-fetched for a "return to the restaurant" method. Examiners should have seen mud or footprints on the umbrella, unless Banba really did walk barefoot. But regardless, it would be too troublesome to use such a complicating method. I don't even think the umbrella was used in his attempt to go back inside the bathroom window since my first argument stated that the wind was heading due NW (the wind would have little effect on the umbrella if it were to be placed back outside). I do, however, believe it is possible that Banba used it for his murder trick. Not sure how though... But like you said, in order to obscure his footprints, he probably used the supplies in the car.
CALLING OUT BANBA: In your other post, you claimed that Hatsune probably called out Banba (to tell his that the marriage is off and that they're related). But in the manga, someone called Hatsune upon her departure from her car. Right after she answered her phone, there was some explosion of noise (Hatsune was attacked?). You could say, "Maybe she called him beforehand?" But if that were the case, then she would have time to explain, then and there, that the two were related. I think we both believe that Banba doesn't know the circumstances yet at this point since Conan pointed out that a "sad truth" will soon be revealed. So for what other reason did Banba have to kill the bride? I'm sure if Hatsune called Banba and told him that the marriage was off, Banba would question why first, rather than hang up the phone and have the urge to kill her before she could explain that they are fraternal twins.
There is still a problem I am flustered with... How did no one manage to see Banba attacking Hatsune in the first place? Her car is right outside the restaurant and there are multiple large windows inside the building. There should have been onlookers that would have seen, if not heard, the attack. The windows don't have one-sided glass (people in the restaurant could see through it perfectly). Were the guests that occupied with themselves? Was there some distraction used by the culprit? I'm so focused on this problem that I can't even concentrate on other important ones...
By the way, I liked that article you posted. The study, about how the height of twins and singles differentiate, perfectly explains why Conan asked Ran about Hatsune's height. I applaud you for finding this article.
(Sorry it took me so long to post this. I accidentally closed my browser, so I had to type up this post all over again from scratch )
EDIT: Nevermind... Saw the spoilers...