There Is a 7-11 at fenqihu stop over enroute to alishan.
Where is the 7-11 that you mentioned about alishan train station?
There is a 7-11 near FQH for break.
Alishan got 2 7-11, 1 at the small bus terminal where you board the bus, alight I don't know. Another is near to Alishan train station, you will see it after walking down the trail.