I don't understand the part of merging the script hash with another hash to form a redeem script.
Do you mean in the code he provided for P2SH version?
It's just for the generation of P2SH address which is not used to spend the transaction, he did that to conveniently create an output with that script.
The Redeem Script is already the "
dumb_script=7551".
This is literally the instruction to spend it: He used "
OP_1" to fulfill the RedeemScript "
OP_DROP, OP_1"
Comments: Used "OP_1, OP_DROP, OP_1" as the sigScript, which, including length prefixes, is 0x0451027551 - all it took.