In which mode were hymns originally composed?

Christ is risen!

Seeing this article about Theotokos and Virgin being originally composed in soft chromatic forth mode got me wondering in which mode were various hymns originally sung in. In other words, which is the most traditional/old way to chant a hymn in? I would have liked to browse the Greek Psaltologion to find my answers but my knowledge is limited to using Google Translate :)

O Gladsome Light: second mode ending on Di or ending on Vou? In most Romanian chant books this hymn is composed in the latter.
Saturday Vespers Prokeimenon: second mode or plagal second mode? Chant books in Romania indicate one or the other but the melody is mostly the same.
Monday Vespers Prokeimenon: legetos or soft chromatic forth mode? Here in Romania we chant it in legetos but in Fr. Ephraim's scores it's composed in the latter.

Douloi Kyrion: Chanting it in all of the eight modes seems to be a new kind of thing. In which mode was it originally sung in?
By the Waters of Babylon?
Logon Agathon: legetos or agia?
Let every breath (before the gospel): second mode or the mode of the antiphonon?
Sunday Theotokion before the Doxology: second mode or the mode of the doxastica?

Typika Second Stasis: I've seen scores in grave mode and legetos but Petros Lambadarios composed it in second mode with each verse ending on Vou.
Come let us worship: second mode ending on Di or on Vou? In old printed books and manuscripts it's composed in the latter.
Anaphora: "I Will Love Thee..." and "Father, Son..." in grave mode from Ga and the rest in second mode? In an old manuscript "Father, Son..." is composed in plagal fourth triphonos which may indicate it is meant to sound like grave mode.
One is Holy: recited in kliton or second mode? I do both ways.
We have seen the True Light: second mode ending on Vou or ending on Di with a conclusion/final cadence formula?
Let our mouths be filled: second mode or nenano? I saw an article stating that traditionally it was sung in nenano not second mode.
I will bless the Lord (for the Presanctified Liturgy): second or plagal second?
Any hymns I missed?

I would be grateful if we could have a discussion about this subject. This would be very helpful especially for a beginner-to-intermediate psalte like me (who is also a Transylvanian - a region where mostly everything is chanted in plagal first :LOL:) wishing to chant traditionally.

In Christ,
As far as oldest way to chant things, we don't really know how things were chanted in the 4th century, and hardly even in the 12th, although there are those whose knowledge of older methods of chant goes back much farther than mine. Simon Caras claims to have reconstructed an older tradition of chant, but his reconstructions are very controversial and probly wrong. However I can answer most of your questions in terms of the living tradition of the Ecumenical Patriarchate, which preserves most carefully the oldest living tradition of chant.

φως ἱλαρον-di
Saturday Vespers Prokeimenon--Plagal second
Monday Vespers Prokeimenon--The music I have says second, but it could be fourth chromatic. I'm pretty sure it's not legetos, though. I'll have to double check that one, though.
δουλοι κυριον--in a brief search I find Classical melodies in Plagal 1 & 2, agia, and plagal fourth, perhaps more (Petros, Chourmouziou, & one other). I can't say which is oldest. No Koukouzelis
λογον ἀγαθον--I find Classical melodies of Phokaeos in Pl 4, and Pl 1 Petros. I also find Chourmouziou in Legetos.
πασα πνοι-second
τυπικα-second, although Varus and Legetos might not be wrong--I just haven't ever seen them. Second is most traditional, however.
δευτε προσκυνησωμεν--di. However I would be curious to see which books have bou.
αγαπησωσε--the argon version done at the EP is in second. There is an alternate, although less common, version in Varus.
πατερα υἱον--the same as the rest of the leitourgika. Chanted if there is only one priest.
λειτουργικα--Most traditionally, and still for the liturgy of St. Basil, it is done in Second mode. Generally at the EP they chant it in Plagal first Patriarchal or Kliton. Occasionally Plagal 1 Frygios (I think--I'll have to double check). If the Patriarch is not celebrating, even at the Phanar they will chant liturgy in all eight modes, so I am told.
εις ἀγιος--Kliton
ειδομεν τὸ φως--di
πληρωθητω--Never heard of it in Nenano. Sounds unlikely. Second makes more sense anyway.

Let me know if this makes sense or if I missed something. I'd be happy to chat as well.