rapid 'single-fire' twitches sound to me like myoclonus not fasciculations. Myoclnus in the setting you describe is usually benign.
Fasciculations in ALS may fluctaute but would not for instance completely disappear (unless the muscle was totally wasted) and would tend to spread to different areas over time
weak means at least a decreased inability to move the muscle against resistance, and/or a difference from side to side, without other explanation ie pain etc
muscle fatigue is usually not true weakness from a neurological/nerve cause, but a problem with the muscle itself may cause early fatigue (but could be anything from normal to a myopathy)
Foot arch abnormalities could develop later in MS due to severe disability, but would not be an early or only feature
Fasciculations in the absence of weakness or atrophy is not suggestive of ALS
BFS is much more common than ALS
Good luck
Also:
8. What does atrophy look like in the early stages? Would a patient be able to identify it?
Again, thank you for your time.
-andamin