Utreexo makes it possible to use Bitcoin securely without downloading the full blockchain. Since some prominent Bitcoin Core devs think there are not enough full nodes, I speculate that some may not like the prospect of less full nodes (less decentralization), and further more, more code complexity.
Also, Utreexo is still in development.
Also, since it ...
Bitcoin Core is not required to do anything, but it is a useful software that you can use instead of trying to reimplement everything it does. Or you could use some other software that does the same thing. However Bitcoin Core is by far the most used and reviewed software compared to other node and wallet software.
You can use Bitcoin Core to create wallets ...
This is specified in BIP 13: https://github.com/bitcoin/bips/blob/master/bip-0013.mediawiki
You compute the Base58Check encoding of the P2SH version byte (5 for mainnet, 196 for testnet) followed by the 20-byte Hash160 of the script.
The code you're quoting is for P2PKH addresses, not P2SH.