Prelude of Maalai Yen Vedhanai from Sethu

Recently I noticed and tried to understand the composition of the piano prelude of the song Maalai Yen Vedhanai Koottudhadi from Sethu. The song is in D Major scale (shankarabharanam) and the notes of the pallavi go like:

GaMaGa  GaMaGa  GaPaSaNi.
maalaiyen  vedhanai  koottudhadi

So Ilayaraja has tried to play on the GaMaGa (F#GF#) tune for the prelude. The piano piece opening is similar to the 'maalai yen vedhanai' tune. But since it talks about the pain of love and also because of the scene before this song, he brings in a melancholic and contemplative mood in the prelude. He does this by employing different chords that predominantly seem to be in its relative minor scale (B minor scale) and doesn't uses the tonic chord of the D Major scale at all. He also uses the Dha1 note (A# accidental note in D major scale) in some of the chords to bring in a little melancholic feel. Towards the end of the piano prelude (0:13 to 0:15 secs in this video), he creates a small break and the chord used there creates a certain tension. And then he releases the tension by leading up to the tonic chord (DMaj7th i think) using it for the first time for the GaMaGa tune (at 0:16). Building up this tension and releasing it is my favorite portion of the prelude. I used to keep repeating this prelude alone to listen to it again and again. From a composition point of view, this was really interesting to me.

Listen to this prelude in the video below:

Chords used in the prelude below. I got this by loading the song in a mac based software called Capo 3.

Bm7 Bm7 | Em7 Em7 | GMaj7 GMaj7 |
A#aug Gm | GMaj7 Em7 | F#7- F#7 |

It showed A#7 at one place which I thought was wrong and change to A#aug above.
It aslo showed F#m7 at the end for the chord which creates that tension. This again I think is wrong. I changed that to F#7, but then I am not sure if that chord is correct.