Yes, you can use multiple automations at the same time, but they should not contradict each other.
Yes, if they contradict each other. For example, in one you call to the action to close the ticket, in the other to reopen the ticket.
This will help you establish automated processes and not waste time on unnecessary operations.
No, there is no possibility to copy a worklog from project A to project B.
The "Add watchers" option is not available in this release of the application.
Just the Jira service desk agents are charged.