***** NOT A TEACHER *****
!. I believe that you are correct: "past" is a preposition in some sentences:
I reached past Mona [my hand extended beyond her] and took her glass of soda. She said, "That's mine!" I replied: "Too bad! I'm very thirsty!"
2. Here is a secret: whenever you want to see how authors use the English language, go to the "books" section of Google and type in the words that interest you. So I went there and typed in "reached past." There were many examples of how the two words are used. Here are just two examples:
a. "He stalked [approached] her, then to her complete surprise, reached past her and grabbed the brush."
-- Wild at Heart (2011) by Susan Fox.
b. "The three entered the kitchen and Aleksandr reached past Abigail to snag [get] the last of the cookies ahead of Jonas."
-- Oceans of Fire (2005) by Christine Feehan.
Student or Learner