• document/comment your code. In the code.
• re-checking .next in the second loop is unusually defensive
• do not have a comment repeat what a statement "does" (reset node to point to head):
  have it illuminate what is achieved/what-for/why something is done (the way it is)

(Eyeballing remove_middle_node, it uses O(1) additional space and Θ(n) time.)

• document/comment your code. In the code.
• re-checking .next in the second loop is unusually defensive
• do not have a comment repeat what a statement "does" (reset node to point to head):
  have it illuminate what is achieved/what-for/why something is done (the way it is)
• the ruby way to have a statement executed, say, middle times may be

  middle.times do
      node = node.next
  

(Eyeballing remove_middle_node, it uses O(1) additional space and Θ(n) time.)