At this time, Fiverr CS is taking longer than usual (sometimes 10 days, if not longer) to get back to a ticket, so in that regard you will need to wait. But at this stage, I don’t know if there is much they could really do beyond making sure this order doesn’t impact your order completion rate. Even then, there’s no guarantee they’ll do so.
If I were in this situation, I would have delivered the original agreed-upon scope of the order. That is what this buyer ordered, so that is what they would receive. If they refuse to pay for additional aspects, then that is on them, as I wouldn’t give them away for free. I don’t think you should have cancelled, and if I’m reading this correctly there is a misunderstanding: a buyer or seller initiating the cancellation doesn’t matter if both parties eventually agree. Either instance will hurt your completion rating, and if they had initiation a cancellation it would have still impacted you.
Did the buyer ask for additional requirements right off the bat after the order was submitted, or after you had already worked on the order? Either way, you shouldn’t have cancelled in my opinion. If he didn’t want to pay for additional services, then you should have delivered what was originally agreed upon. Simple as that. Granted, he could have left you a nasty review, but in my opinion getting paid for the work you did is far more important than one review (and of course, you could review the buyer in turn and leave a comment on their review).
All in all, there really isn’t a “system” to where you could have both benefitted. It just comes down to this being a client who should have understood that they needed to pay to receive additional services, and you should have put your foot down and supplied what was originally agreed upon so you could get paid. Hopefully this is a lesson moving forward, but from here there’s not much that you can do besides blocking the buyer to avoid future orders from them (if you haven’t done so already). In the long run, your completion rate will go back up, and I seriously wouldn’t worry all that much about it. Focus on your other orders, and complete those! It’ll bring it back up in no time.