Express lanes are faster 95% of the time but depending on how you use them, they might not be a lot faster. When heading northbound and exiting to Express on the left, you either have to sit in line or if you're moderately aggressive, go around the pack and it's not that difficult to squeeze in a few hundred meters before the exit.
For alternate routes through Everett, I would not venture off I-5, especially if you have HOV access. As far as sunday traffic, it's always hit or miss. Some sunday afternoons are busy, some are not. Seattle traffic can really be frustrating and it's only getting worse so enjoy that nice bimmer for its luxury commuting abilities.