Trimming issues can be easily dismissed. Remember Air Alaska DC-9/MD family that spiralled down onto the Pacific after the trim jack broke due to under greasing in maintenance.
I remember that accident, and the factors, and they should be dismissed relative to discussion of a Twin Otter trim system - the two systems are entirely different. A failed Twin Otter elevator control or elevator trim control will result in the elevator more or less trailing, unless a force is being applied to one of the surfaces. The DC-9 Trim is an all moving horizontal stabilizer, with the trim actuator at the front, which with a failed trim jack screw will mot likely go divergent to an extreme of the trimming range - probably instantly irrecoverable.
The second flap interconnecting trim tab is, as stated, there to minimize pitch control forces with flap position changes. Twin Otter flaps are hugely effective, and this additional system is required.
Control system linkages are known to freeze (in the low temperature sense) in several types. That's a bit of a design oversight that it's possible. Other than that, the seizing of an elevator control in a certain position is much more rare than a one side linkage failure. In such a case, the elevator will generally float free in one direction, and can be controlled against a deliberate out of trim selected by the pilot. But this is not something that most pilots will diagnose, and correctly set up in seconds, so is not an instant fix. I have flown an aircraft on two occasions where a massive pitch control rigging error resulted in flying a circuit while applying only variations in high "push" forces on the control wheel, as opposed to being trimmed, or "pulling" for some of the desired pitch changes. This is counter intuitive, and requires a careful thought, which many pilots could not be expected to get right on short final.