As long as you delivered the initial order on time, you do not have to worry about the BIG red LA:TE letters that you see on your order page. You will not receive a late delivery penalty for the order.
Just send your buyer a reminder message on the order page and ask them to send you further instructions so that you could get started on the revision ASAP. If your buyer doesn’t seem to respond to the reminder, create a support ticket to let CS know.
If the buyer is still non-responsive, I’d just redeliver the original files (without making any changes to them) and request for further information. However, before doing that, I suggest that you confirm (from CS) that it’s okay to do that.
Yes. It is possible for your buyer to request for cancellation even after you deliver the revision. However, you can deny their cancellation request.