@LordDiscord
 
It was always possible to remove them. With consequences, if done for a longer time, but possible.  
The whole point of the Season 4 Opener was that Celestia and Luna removed them to defeat Discord, then kept them as convenient weapon, believing everything would be fine, only to cause a disaster with this because it weakened the Tree of Harmony that they removed his energy source for too long and that, before this happened, the Tree lasted for more than 1,000 Years before collapsing.  
Do you even really watch the episodes or do you play videogames while they run in the background?

@LordDiscord  
They removed them.

@Background Pony #CD1E  
How do they even have the elements to begin with? They are suppose to be in the Tree of Harmony and to my understanding, could not be removed again. So how the hell do they even have them.

@Background Pony #CD1E  
How do they even have the elements to begin with? They are suppose to be in the Tree of Harmony and to my understanding, could not be removed again. So how the hell do they even have them.

Really hated this arc; The idea that the elements of harmony somehow won’t work on Discord is pure BS.
 
the elements represent FRIENDSHIP, and brainwashing isn’t friendship. The EOH wouldn’t suddenly not work on Discord just because he’s calling himself accord, harmony’s different than Tyranny, which is what Brainwashing is.

@dynamicv
 
I found this very interesting as well, Celestia is actually not supposed to be able to use the Elements of Harmony again, since they got tied to Twilight and her friends when they defeated Nightmare Moon, yet here she was able to.  
This reinforces my theory that the Elements of Harmony can fulfill certain desires or wishes their wielders have in the way they are working and in what effects they cause on their targets exactly.  
Maybe Celestia’s will to defeat Accord and to restore true order in Equestria was so strong that the Element simply listened.  
Another possibility is that the Element of Magic got partially empowered by the presence of the other, actual Element Bearers.  
Or the Element of Magic simply works differently and can be used by anyone with enough magic power and what Celestia meant when she said that the Elements can’t be used by her and Luna anymore was only that they can’t use all of them anymore. But since the other Element Bearers were present and Celestia only had to use the Element of Magic, it worked.  
There are many possibilities here why she could use it and lots of opportunity to theorize, speculate and analyze.  
Instead of slowing down, the comics/show really become more interesting with each released issue/arc or episode.

Too bad replacing one of the bearers doesn’t work, and gives the Elements a false start (Return of Harmony Part 2, where Spike tries to fill in for Rainbow). Though now we know even Celestia, who has wielded all the Elements by herself before, can’t stand in for Twilight. Accord gambled that she wouldn’t be able too, he was right to say on the previous page that they wouldn’t be able to use them against him without Twilight.
 
Accord still uses the magic in the Elements himself for a spell… can this be done? Well, according to Magical Mystery Cure, it’s possible to tap into the Elements with an external spell even when most of the bearers aren’t present. So probably yes.
 
Also notice how Starlight’s horn isn’t glowing here. The shield that protects her from Accord’s mind control is the one she’s had around her the whole time: the one Celestia made two issues ago.

celestia used the–
 

Okay, I guess the connection still works.

That last panel as they use the Elements is basically their last moment having free will before the Elements end up amplifying Accord’s power and leading to them and everyone else becoming brainwashed moments later. They should have known the Elements would work the opposite way on a being of order compared to Discord.

gol dang Rarity What the hell is wrong?! Dude why are you looking like that. I know you are resistant to bright lights and flashes but this is ridiculous.