no-one? I would really appreciate any help.
what's the mainline in this tarrasch?

I don't know the Tarrasch well, but I think it is more common for the White to play 4. exd5. After 4. ...exd5, the Wiki says that: "The main line continues 5.Ngf3 Nc6 6.Bb5 Bd6 7.0-0 Nge7 8.dxc5 Bxc5 9.Nb3 Bb6 with a position where, if White can neutralize the activity of Black's pieces in the middlegame, he will have a slight structural advantage in the ending." Maybe this can help.
Chess is self discovery. In this age of the internet you can't use that 12 pounds between your shoulders and hunt out an answer?

Thought you meant the Tarrasch variation of the QGD.
In the French Tarrasch, your first variation is prefered, though Watson, in his Play the French 3rd ed. page 83 prefers to play ...cxd and then ...Nc6. It will usually transpose, however. After 6. ...Qb6 he gives:
7. exd5 exd5 8. Nb3 (8. Qb3 Qxb3 9. Nxb3 Nf6 10. a3 Bd6 11. Bd3 0-0 12. 0-0 Bg4! and he evaluates black as slightly better) 8. ...Bg4! (he says ...Nf6 is normal so this may be a TN by him) 9. Be2 Nf6 10. 0-0 Bd6 11. h3 Bxf3 12. Bxf3 0-0 and says black's knights are stronger than white's bishops.
7. Qa4?! Bd7 (threatening Nb4 and Rc8) 8. Bb5 dxe4 9. Nxe4 a6 and again evaluates black as better (from Suechting-Alekhine, Carlsbad 1911).
I always get hopelessly lost here;
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