The Elements of Harmony: Speculation and Apex Artillery

The Elements of Harmony are a very peculiar set of artefacts. Six gemstones that only work when wielded together by a group of bearers that represent their spirits and are connected to them. It is, however, possible for a single pony to do that on its own, Celestia is living proof.
In "The Return of Harmony" Princess Celestia tells us something very important: She and her sister are no longer connected to the Elements, only the Mane 6 can use them now.
This implies two things:
First that there has been a "connection" (probably WLAN ) between Celestia and the Elements and that this connection has been cut off.
Second that Celestia cannot reestablish that connection.
This leads to an interesting question. If Celestia and Luna have been connected to the Elements but cannot cause such a connection, how did it come to be in the first place? Who or what connected the Mane 6 to the Elements?
I personally think that the Elements themselves did that. They were the only thing present.
They have shown some kind of awareness by turning into necklaces with cutie mark shaped gemstones completely on their own.
It is safe to assume that the Elements do somehow "know" their current bearers. After all, they "knew" that the Mane 6 did represent their spirits. If not, anyone who had come in there and "spouted a lotta hooey" could have become a bearer.
So we have established that the Elements of Harmony connect to 1-6 suitable bearers and thereby allow these to use them.
Now, what exactly do the Elements do?
Well, we don't know, but they are shown to be able to perform a set of feats.
I'll list them by order of appearance and mark those that only occur when used together with a 'T'.

1. (T) banishing ponies in the moon for a very very long time
2. levitation (a.k.a hovering)
3. emitting bright light
4. appearing out of nowhere (only confirmed for the Element of Magic)
5. automorphosis (a.k.a turning into jewelry or spherical stones)
6. telekinesis (their bearers float when using them)
7. (T) defeating gods
8. brainwashing
9. (T) turning ponies into stone
10. (T) emitting cutie mark shaped particles
11. (T) altering almost anything on a presumably global level (maybe only reverses magic) (a.k.a World Healing Wave)
12. (T) defeating armies (Assumed by Celestia. She seems to be a reliable source.)
13. (T) turning stone back into ponies
14. altering cutie marks
15. (T) emitting massive beams of magic
16. (T) teleportation
17. (T) turning ponies into alicorns (assuming it was their magic that transformed Twilight)
18. (T) summoning Discord
17. (T) beginning and stopping to be part of the Tree of Harmony

We see that the Elements are really quite powerful, though we do not know how powerful. They are capable of defeating gods in a matter of a minute.
If we assume that the Elements are not without limit, we have to conclude that some feats are more difficult to perform than others.
As an indication for that, we can use the time they took to perform a particular feat:

1. (41s) They restored the Tree of Harmony and undid the Plunder Seeds.
2./3. (38s / 25s) They turned Discord into stone and undid all of his chaos. Celestia and Luna needed less time, probably because he wasn't aware of the threat.
2./3. (29s) They released Discord form being stone.
4./5. (23s-38s ?)They banished Nightmare Moon to the moon. Time is unclear because of idle flying around on Celestia's side.
4./5. (25s) They turned into necklaces, defeated Nightmare Moon and brainwashed her to be nice again.
6. (18s) They summoned Discord from elsewhere to Ponyville.
7. (15s) They launched Twilight Sparkle into space / the aether / the Twilight Zone / whatever and possibly caused her transformation into an alicorn.

Apparently Discord is quite tough. Unsurprisingly, Twilight is not.

Note aside:
Concerning King Sombra: It was neither stated nor implied that the princesses used the Elements on him. The scene showing him being turned to shadow does not involve any characteristics associated with the Elements (rainbows and glowing jewelry).
It also does not involve the shiny bubble and floating gemstones seen in S04E02.
Concerning Equestria Girls: I don't consider it canon. If I did, I would put the Crown of Magic on Discord's head and watch the world burn.

EDIT: Integrated the information from S04E01/2