Longest hypercube path
Husk, 27 26 24 bytes

→foΛεẊδṁ≠ÖLm↓≠⁰←ġ→PΠmṠe¬


Brute force, so very slow.
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Explanation

Husk reads naturally from right to left.

←ġ→PΠmṠe¬  Hypercube sequences ending in second input, say y=[1,1,0]
     mṠe¬  Pair each element with its negation: [[0,1],[0,1],[1,0]]
    Π      Cartesian product: [[0,0,1],[1,0,1],..,[1,1,0]]
   P       Permutations.
 ġ→        Group by last element
←          and take first group.
           The permutations are ordered so that those with last element y come first,
           so they are grouped together and returned here.

ÖLm↓≠⁰  Find first input.
  m     For each permutation,
   ↓≠⁰  drop all elements before the first input.
ÖL      Sort by length.

foΛεẊδṁ≠  Check path condition.
fo        Keep those lists that satisfy:
    Ẋ      For each adjacent pair (e.g. [0,1,0] and [1,1,0]),
      ṁ    take sum of
       ≠   absolute differences
     δ     of corresponding elements: 1+0+0 gives 1.
  Λε       Each value is at most 1.

→  Finally, return last element (which has greatest length).