dialog.js

function addVocalAttribute(id, creditedAs) {
	MB.relationshipEditor.relationshipDialogDispatch({
		type: 'update-attribute',
		action: {
			type: 'update-multiselect-attribute',
			// rootKey,
			action: {
				type: 'update-value-autocomplete',
				// valueKey,
				action: {
					type: 'select-item',
					item: {
						id,
						// ...
					},
				},
			},
		},
	});

	if (creditedAs) {
		MB.relationshipEditor.relationshipDialogDispatch({
			type: 'update-attribute',
			action: {
				type: 'update-multiselect-attribute',
				// rootKey,
				action: {
					type: 'set-value-credit',
					// valueKey,
					creditedAs,
				},
			},
		});
	}
}

/********************************************************************************************************************
 * Equivalent code to add the rel without opening a dialog. Currently causes a sentry error...                      *
 * Based on https://github.com/kellnerd/musicbrainz-scripts/blob/beta/src/relationship-editor/createRelationship.js *
 ********************************************************************************************************************/

/**
 * @param {Object} [options]
 * @param {ArtistT} options.artist The performing artist.
 * @param {string} [options.roleName] Credited name of the voice actor's role (optional).
 * @param {string} [options.artistCredit] Credited name of the performing artist (optional).
 */
function createVoiceActorRelationship({ artist, roleName, artistCredit }) {
	createRelationship({
		target: artist,
		linkTypeID: 60, // performance -> performer -> vocals
		entity0_credit: artistCredit,
		attributes: [{
			type: { gid: 'd3a36e62-a7c4-4eb9-839f-adfebe87ac12' },
			typeID: 561,
			typeName: 'spoken vocals',
			credited_as: roleName,
		}],
	});
}

Setting attributes in a relationship dialog will be simplified by metabrainz/musicbrainz-server#2733.
Now I only have to figure out why createVoiceActorRelationship does not work as expected....

I have created a separate gist with a complete code sample for the second issue:
https://gist.github.com/kellnerd/dc39236e58d5fb39b41a53a883eacb09