Can 2 stage furnace be forced to stay in low

It sounds as if the slot is OK at least on one side of the bend, or perhaps both. If so you can make a shaper out of a piece of wood whose thickness is cut to the width of the slot.

Insert the wood in the good section of the slot and force it through the bent section. You may even be able to tap it sideways with a hammer.

If the wood is too soft to bent the metal back, try a harder piece of wood or even a piece of metal if you can find one of the right size. A piece of wood layered to the right thickness with a thin piece of metal toward the bent side of the track would also work.

Couple options come to mind:

1. Change the fan.
2. Change the fan speed.
3. Change the fan connection to the vent.
4. Dampen the vent.
5. Change the size or length of the vent.

If you can't or don't want to change the fan, then try dampening the vent by clamping it somewhere to the building structure and possibly adding some padding or pipe insulation where it makes those connections. This changes the resonance length without changing the actual length of the pipe.

If dampening the vent doesn't work, then you can change the length of the vent. You mention that you can't make it shorter, but perhaps you can make it longer, or larger. Check the manufacturers requirements on the maximum length, number of elbows, minimum size, and required slope before making any changes like this.