Yes, you can invite other users to your account and decide to be either read-write or read-only permission.
When you define a web check monitoring, it monitors website loading and you can configure it to monitor SSL expiry
You can define how frequency you need to check your website, there are many options, however available periods are available based on your current subscription.
Shopify currently has 3 methods available to send notifications:
Yes, you can define multiple contacts and select the communication preference for each of them
Here are the differences between each plan: