If the crew of an aircraft operating under FAR Part 121 and conducting an instrument approach determines, as they begin their descent from the MDA/DA/DH, that they will be unable to touchdown within the touchdown zone of the runway is a missed approach mandatory?
This is spelled out in 91.175(c), in particular
Operation below DA/DH or MDA. Except as provided in paragraph (l) of this section or § 91.176 of this chapter, where a DA/DH or MDA is applicable, no pilot may operate an aircraft, except a military aircraft of the United States, below the authorized MDA or continue an approach below the authorized DA/DH unless
The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers, and for operations conducted under part 121 or part 135 unless that descent rate will allow touchdown to occur within the touchdown zone of the runway of intended landing
Part 135 pilot: I think there is the practical and legal answer here. The legal answer is yes you must execute a missed approach. That's why the regulation specifically uses the word "continuously." If at any point you realize it's not going to work, you must go missed.
However, if this realization comes late in the flare and you make the determination that landing is the safer choice compared to going missed, then you should obviously land. You have to remember that missed approach procedures are NOT protected for climb gradient once you descend below the MDA/DH and this becomes dramatically more important the lower you are to the surface and the closer to the departure end you are.
A realistic example: Part 135 ops, you're flying a C208 into a shorter runway, lets say 3,000 feet. You realize that you will be unable to land in the touchdown zone due to a sudden wind gust which keeps you airborne during the flare. However, there is a high obstacle (rising terrain, cell phone tower, etc.) near the departure runway end. Depending on visibility and ceilings you may not be able to visually ensure obstacle clearance and can't be assured clearance based on the procedure alone. If you believe that you will make a safe landing due to having high landing distance available vs. landing performance margin, I would continue the landing rather than risk a CFIT. CFITs aren't survivable, but runway overruns usually are.