If we say that "X" has always existed even before when there was nothing, should we say:
i. X has always been there.
ii. There has always been X.
iii. X has always existed.
iv. X has always been eternal.
For the 'iv' what may come in mind is that what 'eternal' mean?
Something that has always been and will always remain.
OR
Something that will always remain.
Does the word 'eternal' cover the existence in past too?
Eternal means , "lasting or existing forever; without end."
Or simply, "always existing".
That covers all the time period, before and after. So yes, the word "eternal" would cover the existence in past too.
The meaning of "eternal" is dependent on context. We might talk about the eternal angels, and mean the angels have always existed. On the other hand, an "eternal flame" is a fire that is supposed never to be put out, we know it must have been lit at some point.
If you are talking about angels, we already know that they are supposed to have always existed, and you can say "The angels are eternal". But if you want to make a point and teach a new idea then "The angels have always existed" is unambiguous.
If you are being philosophical, you can look at the word "sempiternal" which means "always eternal, lasting for ever" in contrast with "eternal: existing out of time". In non-philosophical texts, the words "eternal" and "sempiternal" mean the same.
