"Howard Carter: The Path to Tutankhamun, T. G. H. James, Tauris Parke (2001), (1978).

A comprehensive biography of Howard Carter which is somewhat at odds with Tom Hoving's account. The text reproduces some letters in full which show that some of Tom's conclusions are not quite set in stone. It's also stated that what Tom cited as proof that Howard, Lord Carnarvon, and Lady Evelyn had entered the tomb surreptitiously - a draft of an article written by Carnarvon - actually doesn't mention entering the tomb at all. However, according to writer Tom Lawton (see reference), Lord Carnarvon authored an article that was published in the Times where he did admit they snuck in the first night. Possibly the various authors might - just might - provide us with enough quotes for said articles so we can decide for ourselves.

The best guess is yes, they snuck in. After all, who wouldn't?