I am putting the chapter into more detail in case you didn't understand before (if you did you can just skip this chapter), in this AU Theo and the FSM can use all elements and essences before the merge (yes that includes wind, water, darkness, and amber powers that the FSM did not have in the original series but he and Theo have in this AU); any power seen before Dragons Rising With the exception of Heat, Fusion, Shockwave, Quake, Brute Force, Decay, Swarm, Misfortune, and Fear as in this AU they existed before the merge (dragons rising series) so yes Theo can use them.
I am also aware that the Forbidden Five got their powers and dark energy from Thunderfang, so why can Theo use it? I don't know imagine some random stuff its a fanfic for (---)'s sake, and yes that means Theo can also use Thunderthangs corrupted Energy, Theroxian Magic, and Shatterspin.
Amber is not an artificial element in this AU, this Au's version can allow Theo or FSM to be able to copy any element or essence and keep it permanently, just to be clear only people like Theo and FSM can do this, the reason why Skylor or the past Elemental Masters of Amber couldn't was because they were human, so they can only copy elemental powers and some essences (e.g Garmadons destruction as seen in Hunted) but only have a limited control over it and can only use them for a short amount of time, for humans after a while the imitated elements and essences will be gone and would have to be replicated again to be able to use them; human users such as Skylor cannot copy Golden Power or Darkness.
Golden Power in this AU can allow someone to instantly master martial arts such as Airjitzu, Spinjitzu etc as seen in the story; in this AU if someone has enough mastery of the Golden Power they can utilize Forbidden Spinjitzu and the Spinjitzu Burst regardless of the environment or if they have the scroll, Golden Power is also capable of enhancing Elements and Essences in this AU.
(And on that note, yes Theo can use Forbidden Spinjitzu without the scrolls, he can also use Airjitzu, Rising Dragon Technique, and the Spinjitzu Burst regardless of the environment)
To explain the Time power thing, in the original AU Time Elementals could only pause, speed up, slow down, and reverse time for a few seconds and in a limited range, they could only do more by opening a Time Vortex, in this AU a Time elemental can hold up their element for about an hour and if the user enhances the element with Golden Power it can affect the entire realm, though not any of the others.