offset and max_num_pages in pagination gallery

If you want max_num_pages, you need to use WP_Query instead of get_posts. You can then use $your_query_object->max_num_pages in your code.

offset and paged don’t work together, because both of them ultimately set LIMIT in the MySQL query. offset query parameter will always override paged. If you want to offset the results, you need to do the math yourself, multiply page number by posts per page and add offset, and then only use offset. See Making Custom Queries using Offset and Pagination for more info.