I don't know of any overhead images or maps that illustrate the current situation correctly; Google Earth images are out of date and SDOT's construction maps show the whole Broad Street underpass taken up by westbound car traffic. But on the ground right now the northern half of the underpass is blocked off entirely from car traffic. From the northwest corner of 9th and Mercer, ride west down the curving northern sidewalk until you're past the barrier, then hop off the curb into the empty street for a traffic-free ride straight to Taylor and Harrison. From there it's two blocks to Seattle's famous Bike Squid (I know that's what you came to town for)... and several decent bike routes to Belltown and downtown.
The second one is a permanent bridge over the Interbay rail yard that people just don't know about. Someone recently commented on Seattle Bike Blog that he'd been asked for directions from the Elliott Bay Trail to the Interbay Whole Foods and was stumped for a route across the tracks; you can't really get to the Magnolia Bridge, the Dravus Street Bridge is out of the way and not much fun to ride, the Amgen Bridge is out of the way to the south and requires hauling your bike up and down stairs.
But there's another bridge, just south of the Magnolia Bridge, that gets you over the tracks and, optionally, Elliott Ave/15th Ave. It seems the road it carries is called Galer Street (Galer has to be the most disjointed street designation in Seattle), but what's distinctive about it is that it's shaped like a big S, so I call it the S-Curve Bridge. I imagine it gets lots of traffic certain times of day (it looks like the main motor vehicle access to Amgen and Piers 90 and 91) but much of the time it's almost eerily empty. On the eastbound side of the road there's a shoulder you can use as a bike lane if there's traffic trying to pass you; the westbound side has both a shoulder and a protected sidewalk leading to/from a winding ramp down to the west side of Elliott. If there was lots of bike and pedestrian traffic on the bridge the sidewalk and ramp would get pretty crowded... but then again, it probably wouldn't be any worse than the Fremont Bridge, which the City of Seattle seems to think is showcase bike infrastructure.