The third and final hint tells us that
each pair of lines gives a single letter.
So let's get going!
I'm ambiguous by nature as I grow tall after all
Your puny little earth shall tremble at my call
"I'm ambiguous by nature" because "I" can refer to either the solution of the what-am-I riddle or the letter I, which here are one and the same. "I grow tall after all" ... well, I is certainly a tall letter! The second line sounds very egomaniacal; the speaker sounds like a me-me-me type who would often use the word I.
Also "ambiguous" because a lowercase "L" looks like an uppercase (grown-up) "i".
In small as in big, the extremes I will lead
My address is surely some superstition's seed
(thanks to Al.Sal)
The letter M starts ("leads") the words min and max, which are two "extremes" and synonyms of "small" and "big". Its "address" in the alphabet is 13, which is the seed of many superstitions.
Abnormal my birth in a land of illusion so mortal
You touch the near ground of the specular portal
(that Avigrail does like his mirrors!)
By "touching the ground", a person would form a shape something like n. Doing it "near" a mirror ("specular portal"), we'd find this n reflected to become M. The first line also hints towards mirrors with "land of illusion", and the "birth" of this letter M was certainly "abnormal"!
One long gone monster, not touchable for man
Nary a sinner may tame me as merely nothing can
We need a letter that appears in MONSTER and NOTHING but not in MAN or SINNER. The only possibilities are O and T. Looking more closely, we notice that O appears in every word of the first line except "man" (hence it's "not touchable" for man) but not in any word of the second line except "nothing" (hence "merely nothing can tame me"). So the answer here is O.
A creature of prey hunting for your deceased
I shall turn it soft, into a sustaining feast
The first line sounds like some sort of sinister BEAST. Adding the letter R "shall turn it" into BREAST, which is soft (hey, don't blame me - it was Avigrail's dirty mind, not mine!) and is a sustaining feast for young creatures.
Imprisoned by tradition, tortured to the graves
Royal is my essence, my innards rule the waves
I suspect this may be a dig at the Brits (the OP is German) and their drinking of tea or T! Stereotypically, Brits are "imprisoned by tradition"; they have a "royal" family; and Britannia traditionally "rules the waves".
"Tortured to the graves" as tea is dried out in the sun, cut into pieces and served with boiling water which could, if applied to a human, be seen as torture.
As the elders have foretold, the end is near
And with one difference you can only find hear
I got the answer to this but not the correct explanation. From OP:
"...the end is near and" if you just read on to the next line. "end" and "and" are very similar with only one difference we can also find in "hear" (but not in here).
The keeper of time will choose 2 over infinity
You and me and the devil form a handsome trinity
Infinity is a sideways 8 (as featured in another riddle recently), so let's say "2 over infinity" becomes $2/8$, which is a quarter. On "the keeper of the time" or a clock, this suggests a quarter of an hour, or 15 minutes. Something like a quarter past midnight or 15:00 hours gives us the letter L. As for the second line, "you and me" are the hands forming L while "the devil" must be the second hand (?), the three together forming a "handsome trinity".