...hence non-primary requestors won't receive notifications anymore as they weren't added to Subscribers/CC either.

Out of 570 open/new/stalled tickets in total in RT, there are/were 45 RT issues with open|new|stalled status and more than one requestor:
377,3536,5092,5928,6388,6537,6709,6810,6916,7116,7127,7239,7312,7450,7483,7534,7628,7716,7726,7890,7904,7972, 8004,8050,8184,8215,8268,8270,8309,8334,8416,8417,8480,8599,8643,8681,8913,8942,8985,9002,9020,9022,9033,9052

(Closed tickets = not too interesting.)

If those "non-primary requestor" accounts cannot be added to CC via a script one could probably do that manually within one or two hours.