I believe that there was a reason that Twilight became an alicorn. Celestia didn't only want to see if Twilight had the ability to rise up to the ranks of her and Luna but for another reason. Celestia was grooming Twilight to be the master user of the EoH. The reason she wanted this to happen was not because Celestia no longer had a connection to the EoH but because Twilight understood friendship so well and possibly understood friendship more than Celestia herself. Celestia was actually still connected to the elements of harmony. When she defeated Nmm she defeated an evil subconscious side of luna, she never actually used the EoH on luna but on Nmm. The reason why Celestia chose Twilight to defeat Nmm was because she had no real friends for over 1000 years and her understanding of friendship has gone down since she no longer had anyone to share with it.
Going on there are two very important episodes that tie into the whole entire series. The cutie mark chronicles and Lesson zero. The cutie mark chronicles shows how the mane six got their cutie marks but the most important part was how Twilight got her cutie mark. When Celestia walked over to Twilight she told her that she had never seen a unicorn which such raw magical ability but she has. Star swirled was a great unicorn wasn't he? Was Celestia lying to twilight? The answer is no. Look at the title of the show, " Friendship is Magic". So quite possibly Celestia wasn't talking about her magic ability but her ability to make friends. This is why Celestia said she had never come across a unicorn with such raw ability Star swirled didn't have the ability to make friends. He didn't want to embrace friendship because he thought it was stupid. This may actually be a huge shout out to the world. Not only bronies and antibronies but all across the world. If you don't make friends you'll never unlock your true capacity/ability. If you embrace friendship you can follow your dreams and be happy. If you don't then even if you become successful like Star swirled you'll never become happy because you'll have no friends.
Celestia than mentored Twilight until se got sent to ponyville where she would learn about friendship by herself. Like you said Celestia couldn't interfere with Twilight and one episode justified it. In lesson zero the most important part is the end. Twilight was now free and no longer had to follow a schedule. She now chose to send friendship reports to Celestia rather than doing it as a task.
The last episode I'm going to mention is Magical Mystery cure. This is not about Twilight's destiny since I explained it already but a huge flaw in the episode. Why did they chose that combination. They did everything right except Rd's destiny. Is it really her destiny to control the weather? I have created a format that would suit the episode much better.
Fluttershy would get Rainbow's destiny Rainbow would get Rarity's destiny Rarity would get Applejack's destiny Applejack would get Pinkie Pie's destiny Pinkie pie would get Fluttershy's destiny.
Fluttershy would try out for the Wonderbolts since She can fly and Rainbow dash wants to be in the Wonderbolts Rainbow Dash would sew dresses and try to be with the Canterlot Elite Rarity would try to buck apples Applejack would try to make ponies laugh Pinkie pie would try to control animals
I also realized something when watching the episode. When pinkie was gone the town became grumpy right? So pinkie is sort of an anti depressant for the town but laughter is an antidepressant for Pinkie. Pinkie's destiny was to make ponies laugh so when people stop laughing an ignore pinkie pie she becomes crazy. The one thing she was meant to do for the rest of her life is being ignored so she becomes insane like in Party of one. Also their destinies never changed. The perception of their destiny changed. For apple bloom though the reason why she could perform all of her cutie marks was because she had no cutie mark at the time an she could have an infinite amount of cutie marks.
I think Celestia also gets notably skinnier when entering fury mode, from looking at how slim her hind leg is. Probably burning a whole mess of calories with all that power... now someone bake her some cake before she goes Super Stallion.