Hi, welcome to the forum!
The 30-site plan costs €103 per year with the coupon code START20 (approximately 9,324.40 Russian Rubles). You can upgrade your plan or purchase a subscription anytime from your profile page: https://htprotect.app/profile
Which file did you modify? Isn't there an update-safe way to achieve this with a plugin or extension? Modifying core files is generally a bad idea because those changes are lost on updates. However, the core file check does not affect the security rating ("traffic light").
Yes, if security updates or known vulnerabilities for YOOtheme are published, they will be covered as well.
Yes, you can configure notifications if a site goes offline or becomes unreachable: https://htprotect.app/notifications
Please note that notifications are not sent immediately. A site has to remain unavailable for a certain period to avoid false alarms, as brief outages of a minute or two are quite common.
If you need immediate notifications, you can use a service like UptimeRobot alongside HTProtect.