I must be able to script registering a merge driver. Can this configuration artifact

[merge "my-custom-driver"]
	name = A custom merge driver used to resolve conflicts in certain files
	driver = my-merge-tool.sh %O %A %B

be somehow created in the configuration using a sequence of git config CLI commands? I don't see a documentation anywhere on how to access values in sections named like [merge "my-custom-driver"] rather than with a single unquoted word like [user].

Your mergetool-setup.sh overwrites any existing config entirely, which is undesirable.