Why does the plugin create more than 1 sitemap?
Mobile Sitemap Pro plugin creates multiple sitemap files in two scenarios
Sitemap file should not be to heavy.If it is too heavy then Google Bots will not like it.This will impact your site indexing. So Mobile Sitemap Pro includes only 1000 URL per sitemap.
Mobile Sitemap Pro creates separate sitemap file for eCommerce plugins. Your post and pages are included in a single sitemap file. The other sitemap file contains eCommerce pages only.If you are not using eCommerce plugins then only a single sitemap file is created.
The index sitemap file includes the regular sitemap files.You should submit this index sitemap file to Google.
Which sitemap map should be added to Google?
You should always add the index sitemap file for example your-domain.com/sitemap-index-mobile.xml
Can I use your plugin with regular sitemap plugin?
Yes you can this plugin with regular sitemap plugin.Mobile Sitemap includes only AMP pages.
File is not writable error
If you are getting file not writable error then make sure your web root is writable. You can create below blank files and make them writable (664 file permission)