Longest *shortest available* fixed sequence of moves leading to checkmate?

Question

What is the longest fixed sequence of consecutive moves you can find in any legal position that

1. guarantees checkmating the opponent regardless of their responses (whether by forcing particular responses or otherwise), AND
2. is the shortest available fixed sequence of consecutive moves that checkmates the opponent in that position?

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Example

Have a look at the position below:

[White "Checkmater"]
[Black "Checkmatee"]
[FEN "r5r1/1pq1k1Bp/3pb1p1/1p1P1p2/8/5N2/PP2QPPP/R3K2R w - - 1 18"]

1.Qxe6+ Kd8 2. Bf6+ Qe7 3. Qxe7+ Kc8 4. Rc1+ Kb8 5. Qc7+ Ka7 6. Bd4+ Ka6 7. Qb6#

In theory White has M5: 1.Qxe6+ Kd8 2.Bf6+ Qe7 3.Bxe7+ Ke8 (3...Kc7 4.Qxd6+ Kc8 5.Rc1#) 4.Bf6+ Kf8 5.Qe7#. But White's third move has to be different to achieve these mates: no one move works in both cases of 2...Ke8 and 2...Kc7. (For example, try 3.Bxd6+: with this, 2...Ke8 3.Bxd6+ comes with M2, and 2...Kc7 3.Bxd6+ comes with M5, yet neither of the two mates can be achieved by a fixed sequence of moves.)

However, White has the fixed sequence of moves shown above which guarantees checkmate (satisfying condition 1): 1.Qxe6+ Kd8 2.Bf6+ Qe7 3.Qxe7+ Kc8 4.Rc1+ Kb8 5.Qc7+ Ka7 6.Bd4+ Ka6 7.Qb6#. At 7 checkmater moves long this also appears to be the shortest fixed sequence available in the position above which guarantees checkmate (satisfying condition 2) - for example, we can create longer but equally forced sequences that also satisfy condition 1, such as trivially extending the mate above for example with 3.Qxe7+ Kc8 4.Rc1+ Kb8 5.Qc7+ Ka7 and now instead of 5.Bd4+, play e.g., 5.Qa5+ Kb8 6.Qc7+ Ka7, etc., and thereby add a few moves to lengthen the sequence while keeping it fixed and mate certain at the end.

Note that there is no necessity (in principle at least) for the opponent's moves to be forced. Condition 1 is satisfied as long as - no matter what Black chooses, if they have any choice - the same continuation for White (the checkmater) still results in checkmate.

Above I showed a position where the shortest fixed consecutive series of moves by White that guarantees checkmating Black is 7 moves long. (Not that I claim this as any kind of record.) I would like to see positions where there exists a sequence that satisfies these criteria and where that sequence is as long as possible - I'm guessing 8,9 White moves is small fry for some of the people here.

Answer 1

In other words, you wish for the record length for a completely dual-free sequence. Every White move must be absolutely unique to any possible by Black. Shorter, dualed mating sequences do not count.

To that end, I remembered a mate in 61 from this page on SuperProblem.ru (a Russian based chess problem site). It is also in the Die Schwalbe Chess Problem Database. This works since Black has only one legal move for the whole sequence, thereby guaranteeing that White's mating plan cannot be interrupted or prolonged.

[Title "Bosko Miloseski, SuperProblem.ru 1/24/2020, Dedicated to the participants of TT-235, Mate In 61"]
[FEN "4Q3/1ppr4/br1k4/1p1p4/1PpP1p2/KBp2p2/2P2P2/8 w - - 0 1"]

1. Qe5+! Kc6 2. Qe6+ Rd6 3. Qe8+ Rd7 4. Ba2 Kd6 5. Qe5+ Kc6 6. Qe6+ Rd6 7. Qe8+ Rd7 8. Bb1 Kd6 9. Qe5+ Kc6 10. Qe6+ Rd6 11. Qe8+ Rd7 12. Ka2 Kd6 13. Qe5+ Kc6 14. Qe6+ Rd6 15. Qe8+ Rd7 16. Ka1 Kd6 17. Qe5+ Kc6 18. Qe6+ Rd6 19. Qe8+ Rd7 20. Ba2 Kd6 21. Qe5+ Kc6 22. Qe6+ Rd6 23. Qe8+ Rd7 24. Kb1 Kd6 25. Qe5+ Kc6 26. Qe6+ Rd6 27. Qe8+ Rd7 28. Kc1 Kd6 29. Qe5+ Kc6 30. Qe6+ Rd6 31. Qe8+ Rd7 32. Kd1 Kd6 33. Qe5+ Kc6 34. Qe6+ Rd6 35. Qe8+ Rd7 36. Ke1 Kd6 37. Qe5+ Kc6 38. Qe6+ Rd6 39. Qe8+ Rd7 40. Kf1 Kd6 41. Qe5+ Kc6 42. Qe6+ Rd6 43. Qe8+ Rd7 44. Kg1 Kd6 45. Qe5+ Kc6 46. Qe6+ Rd6 47. Qe8+ Rd7 48. Kh2 Kd6 49. Qe5+ Kc6 50. Qe6+ Rd6 51. Qe8+ Rd7 52. Kh3 Kd6 53. Qe5+ Kc6 54. Qe6+ Rd6 55. Qe8+ Rd7 56. Kg4 Kd6 57. Qe5+ Kc6 58. Qe6+ Rd6 59. Qe8+ Rd7 60. Kf5 Kd6 61. Qe6#

Answer 2

[FEN "k4b2/P1p1p3/KpP1Pp2/1P3P2/3pBp2/3P1P2/8/8 w KQkq - 0 1"]

This may be more "in the spirit": White has a forced 12# by 1.Bd5 2.Bb3 3... 12.Bb7# and Black a ton of variants (didn't count...) but White doesn't care about Black's moves.

EDIT: To increase the number of subgames, all black pawns except c7/b6 could be bishops. (If they move too early, White has a shorter mate, but this is hardly relevant, since Black tries to avoid shorter mates in a problem by definition.)