Longest sequence of forcing moves? What is the longest fixed consecutive series of moves for one player such that the other player's response is forced every move? Assume draw rules must be obeyed (threefold repetition, 50-move rule, etc.).

Note that the moves are only forced for the opponent or 'forced' player; for the forcing player, there is no requirement their moves be forced (as in Longest sequence of mutually forced moves). The forcing player's moves need only be a fixed predetermined sequence to which every response of the opponent is forced.
The sequence ends when a) the forced player plays their final forced response, such that whatever the forcing player does next move, the forced player will have a choice of multiple replies; or b) the game terminates (checkmate, stalemate, draw by threefold repetition, draw by 50-move rule, etc.).
The longest possible series of forcing moves that I have cooked up is 3108 moves long, abusing the 50 move rule to its fullest. Throughout the entire sequence, Black has only one legal move as stipulated.
Firstly, we start with this position.
[FEN &quot;1N6/3r4/3p1p2/3B1n2/5b2/4QNPk/PPPPPP1P/R1B1K1nR w Q - 0 1&quot;]

1. Nxg1+ Kg4 2. Qxf4+ Kh5 3. Qxf5+ Kh6 4. Nxd7 Kg7 5. Qxf6+ Kh7 6. Be4+ Kg8 7. Nb6 d5 8. Bxd5+ Kh7 9. Bg2 Kg8

White sets up the Black king's moving zone. It comes with captures to gain 8 extra moves. Next, White makes a pawn move or sacks a piece on g8/h7 every 50 moves. Getting rid of the light-squared bishop, both rooks and knights, and five promoted pieces, from the a2 through e2 pawns, allows for 10 50-move rule resets. Promoting these pawns allows for 5*6=30 more resets. Thus, there are (10+30)*50+8=2008 moves already.
Afterward, this position is reached. During the 49 move intermission before the next reset, White moves the Black king.
[FEN &quot;6k1/8/5Q2/8/8/6P1/5P1P/2B1K3 w - - 0 1&quot;]

1. Bb2 Kh7 2. Ke2 Kg8 3. Qg6+ Kf8 4. Qh7 Ke8 5. Qg7 Kd8 6. Qf7 Kc8 7. Qe7 Kb8 8. Qd7 Ka8 9. Ke1 Kb8 10. Be5+ Ka8 11. Qc6+ Ka7 12. Bc3

Here, White has 4 pieces to sacrifice along with 17 more pawn moves. This allows for 17+4=21 more resets. As such, the total is now (21*50)+2008=3058 moves. There are no more resets after this.
Finally, White merely plays around for 50 more moves. Black then claims a draw by the 50 move rule. It is important to note that sacking the queen actually loses a ply, as then the position is a draw to the &quot;Dead Position&quot; rule. Checkmating and stalemating also lose the essential last Black ply.
[FEN &quot;8/k7/2Q5/8/8/8/8/4K3 w - - 0 1&quot;]

1. Kf1 Kb8 2. Kg1 Ka7 3. Kh1 Kb8 4. Kh2 Ka7 5. Kg2 Kb8 6. Kf2 Ka7 7. Ke2 Kb8 8. Kd2 Ka7 9. Kc2 Kb8 10. Kb2 Ka7 11. Ka2 Kb8 12. Ka3 Ka7 13. Kb3 Kb8 14. Kc3 Ka7 15. Kd3 Kb8 16. Ke3 Ka7 17. Kf3 Kb8 18. Kg3 Ka7 19. Kh3 Kb8 20. Kh4 Ka7 21. Kg4 Kb8 22. Kf4 Ka7 23. Ke4 Kb8 24. Kd4 Ka7 25. Kc4 Kb8 26. Kb4 Ka7 27. Ka4 Kb8 28. Ka5 Ka7 29. Kb5 Kb8 30. Kc5 Ka7 31. Kd5 Kb8 32. Kd6 Ka7 33. Kd7 Kb8 34. Kd8 Ka7 35. Ke8 Kb8 36. Ke7 Ka7 37. Ke6 Kb8 38. Ke5 Ka7 39. Kf5 Kb8 40. Kf6 Ka7 41. Kf7 Kb8 42. Kf8 Ka7 43. Kg8 Kb8 44. Kg7 Ka7 45. Kg6 Kb8 46. Kg5 Ka7 47. Kh5 Kb8 48. Kh6 Ka7 49. Kh7 Kb8 50. Kh8 Ka7

At last, we have our total sequence of 3158+50=3108 forcing moves.