Yes, JEMH should support both collection of 'involved' party email address in CSV style within a text CF, addresses matching existing Jira users get stored in a multi-user picker CF type as Users.
JEMH does support auto user-creation, and auto-joining a nominated group (your case a) or alternatively through a defaultReporter you define (your case b).
I am not sure if you can extract sender and populate a customfield without some coding but have a look at JEMH. It certainly supports creation of issues with customfields, so you can probably use that approach.
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