I am not sure if you are talking about a seller being overdue but still completing the gig, or if you mean a seller getting a late cancellation and automatic bad review.
If you are the buyer and the seller delivers a gig on time and you just don’t respond before the gig is marked complete, the seller is not harmed. If the seller delivers late because they were waiting for information from you, the seller is technically considered late but they still get paid and you can leave an excellent review. This would have to happen many times to cause any problem for a seller.
If the seller delivers days late, then a buyer has the option of still accepting the delivery and reviewing it positively. You also have the option of clicking the cancel button since the delivery was late. If you do that, the seller does not get paid and an automatic 1 star negative review is left by Fiverr which can lower the sellers rating a lot.
I’m not sure how are unfair from a buyer viewpoint since you have almost full control over the situation. Even if you wait a week to mark the gig complete, as long as you don’t cancel everything is fine. As far as your other issue, Customer Support is easy to reach. The link is now listed in through this thread. You can also email them at firstname.lastname@example.org but I don’t recommend it since you won’t have a ticket number for tracking. If you need additional help on how to submit a ticket, it is explained in more detail here: http://forum.fiverr.com/discussion/how-to-submit-a-support-ticket/