Hi,
1. There is no problem. I recommend you enable Header Builder, it’s easy-to-use.
2. Yes, if you use child theme so it won’t lose when you update theme. However, always make a backup before updating theme: https://hungpd.name/how-to-migrate-or-backup-a-website-with-all-in-one-wp-migration-plugin/
3. You shouldn’t pay attention to the version that is written in your child style.css file. You can write any version you want there. You need update only parent theme.
Also, Please note that theme developers are not responsible for custom code placed in the child theme and additional customization is outside the scope of our support (check the item support policy https://themeforest.net/page/item_support_policy ).
Regards,
Hung PD