Untested: convert author_name query to a tax query for co-authors-plus

rest-cap.php

<?php

add_action( 'rest_api_init', function() {
	global $coauthors_plus;
	if ( ! method_exists( $coauthors_plus, 'get_coauthor_by' ) ) {
		return;
	}

	// When using co-authors-plus, convert author queries to tax queries
	add_action( 'rest_post_query', function( $args ) use ( $coauthors_plus ) {
		if ( ! isset( $args['es'] ) || true != $args['es'] ) {
			return $args;
		}

		$authors = [];

		if ( isset( $args['author__in'] ) ) {
			$author_ids = $args['author__in'];
			$args['author__in'] = [];
			foreach ( $author_ids as $author_id ) {
				$author = $coauthors_plus->get_coauthor_by( 'id', $author_id );
				if ( is_a( $author, 'WP_User' ) ) {
					$args['author__in'][] = (int) $author;
				} else {
					if ( isset( $author->ID ) ) {
						$author = get_the_terms( $author->ID, 'author' );
						$author = wp_list_pluck( $author, 'term_id' );
						$authors = array_merge( $author, $authors );
					}
				}
			}
		}

		if ( isset( $args['author_name'] ) ) {
			$author = $coauthors_plus->get_coauthor_by( 'user_nicename', $args['author_name'] );
			if ( is_a( $author, 'WP_User' ) ) {
				$args['author_name'] = sanitize_user( $args['author_name'] );
			} else {
				if ( isset( $author->ID ) ) {
					$author = get_the_terms( $author->ID, 'author' );
					$author = wp_list_pluck( $author, 'term_id' );
					$authors = array_merge( $author, $authors );
				}
			}
		}

		if ( ! empty( $authors ) ) {
			$tax_query = [
				'taxonomy' => 'author',
				'field' => 'term_id',
				'terms' => $authors,
				'include_children' => false,
			];

			if ( empty( $args['tax_query'] ) ) {
				$args['tax_query'] = [
					$tax_query,
				];
			} else {
				$args['tax_query'] = [
					'relation' => 'AND',
					$tax_query,
					$args['tax_query']
				];
			}
		}

		return $args;
	}, PHP_INT_MAX );
} );