Arggghh! You are going to be traveling on a very, very busy weekend. You are hitting both national holidays -- Canada Day on July 1st and Ind Day on July 4 and the border crossing will be busy and potentially very slow. That is the biggest problem with driving back to Seattle from Vancouver. You can make a reservation on the BC Ferries run from Swartz Bay (and if you do decide to take this route, you absolutely must make a reservation or potentially sit in the parking lot through several sailings).

There is one other option.... I assume you want to travel with a car? You can take the MV Coho from Victoria to Port Angeles and drive down the Olympic Peninsula to Seattle. I think this will be your best bet, to be honest. This is very convenient because it leaves from Victoria's Inner Harbour, basically right in the heart of downtown, so no long drive to Swartz Bay. The drive through Port Angeles and Port Townsend etc is very nice, also. And the Customs and Immigration clearance will be faster than at the highway border crossings.

Let me check on a couple of details further and I will update more....