The 'end when' condition in Q2 is incorrect and leads to typing errors:

  end at $e when $e[self::h2] 

Clearly the keyword "at" should be replaced by "next"
so that the window contains elements strictly between two h2:

  for tumbling window $w in $seq/body/* 
    start previous $s when $s[self::h2]  
    end next $e when $e[self::h2] 
  return ...