Gabrielle covered the first two.
The third question is worrying - "populating data tables used by JSD" implies you might want to write data to the database outside JIRA. Don't. It won't work and you'll break it.
If you are just questioning the licence scheme, for JSD, there are three types of user:
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No. Only Agents respond to JSD issues. They can invite collaborators in to help, but those people are not Agents, and they don't get full Agent abilities just because they're invited in.
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Remember that there have been changes in the model of licencing between JSD 1 and 2. Also between JSD 2 and 3 the role of Collaborator has been replaced with Service Desk Team role to refer to the users who will help resolve an issue.
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The JIRA Service Desk is just an add-on of JIRA so it entirely depends on what JIRA version do you have. Take a read here (https://confluence.atlassian.com/jira/supported-platforms-207488170.html).
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