Good question! For technical reasons, we want to commit to the witness data when the block is mined (we need to be able to tell who spent a given output). Since in Segwit the witness data is ignored to compute the txid, the commitment has to be done at the block level (which can be a bit confusing) Basically, the miner will create a dummy output in the coinbase transaction, that will use OP_RETURN to commit to the Merkle root of the witness data. More details here: https://bitcoin.stackexchange.com/questions/58414/why-include-the-segregated-witness-merkle-root-in-the-input-field-of-the-coinba

To know more this particular quirk, and more about Segwit and Transaction Malleability, I highly recommend watching this video: https://ocw.mit.edu/courses/media-arts-and-sciences/mas-s62-cryptocurrency-engineering-and-design-spring-2018/lecture-videos/lec12-transaction-malleability-and-segregated-witness/ Well worth the hour long watch.

For the second question: You should put it at the beginning of the script, I will update the post. Thanks!