If you receive an order with no instructions, there is no timer starting or running out yet, because the order is “incomplete”, meaning the timer cannot start until your buyer provides the requested instructions
There is also no late delivery issue if an order is incomplete, because again no timer will start until you receive the instructions.
The buyer has nothing to exploit in this case… sending the instructions at a later time will simply make them wait more because of their own fault. As soon as they give you the instructions, your timer will start. IF, again: IF the buyer sends the instructions after a few days and they wants to cancel because “it took you too long”, then you should explain them how the platform works and that it was their fault, not yours, for not providing your instructions on time and for not responding back to your nudging. If they get angry, call out to CS and explain the situation, CS can even contact the buyer in some extreme situations
There’s also no reason to cancel, yet… if that order remains incomplete for a year or so, then you can request CS to cancel it (I’ve had orders remaining incomplete for 2-3 years!)
And finally, cancellation rate is not visible on gig pages or profile pages, it’s for your eyes only and for Fiverr’s internal eyes.
P.S. you could try, however, sending them a message via Fiverr Inbox and ask them if there’s something you can help them with, and remind them that they haven’t provided the info you need to start their order.