Perhaps I don't understand this either. But:

After pressing 'OK'; and then clicking on 'Apply' again (to re-open the 'Apply Walk'); the the 'End Frame' value is automatically recalculated, based on the existing value for 'Cycle repetitions'.

I agree, the interface is confusing. But, I think the designer expects the User to either:
a) Set how many repetitions they want,
OR
b) Select the End Frame.

As far as I understand, the uncontrollable jerkiness only occurs if the User sets the 'End frame' to be higher that its original calculated value.

If the 'End frame' value is set lower than the original value; then the animation stops at that place. Sensibly, I think.

I recommend reporting the missing helpfile text to SM.