Sorry to hear the team can’t get into the same tooling. And I’m sad to hear that you must “comply with Agile development” while leaving the verification team outside the development team. From long ago, I remember how painful it is for both developers and testers when they aren’t working together daily. Back when those problems were more fresh for me, I wrote about software tools that solve for this kind of synchronization:
If you still think writing your own is the way to go, then you will want to use the ASSIGNABLE_USER permission as a filter, instead of CREATE_ISSUES. Technically, Apps can be given permission to be assignees, but that’s a rare corner case. As you build the integration, you should use the permissions API that I linked above to see what other permissions make sense as a filter.
Also, you might find emails easier to map because display names are not guaranteed to be in a consistent format (because of profile visibility settings). The Email API does require getting permission, but should be straightforward for your in-house scenario: