I don’t know how to split versions between testers automatically, but you can download older versions of the application from TestFlight.
On the permissions page for a specific assembly (from the control panel, select the assembly, then select the application assembly, after which you click the "Next" button next to the last assembly to open the others), there is a section called "Share" this assembly, which provides direct download link. If it were me, in this case I would have avoided the automatic notification system TestFlight and sent notifications manually, including the corresponding direct link for each tester. If your delivery system allows this, specify an email address to respond to the feedback page for the assembly, and testers can still send feedback by replying to your (current) message with a message.