Hi Akasha, sounds like you are on the right track.
Yes it sounds like a seized or jammed linkage.
I went out and took some pictures of mine.

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The pedal could be seized where the rod goes through the chassis?
Also check the pivot on the other side.


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From the pivot the a rod runs to the linkage below at the rear of the mower, where you were looking with your uncle.

Just a series of elimination checking that they all move freely.
If it is working you can see them all moving when rock the pedal to and fro.
[Linked Image]


Happy is he who penetrates the mystery of things.