I live on the westside and have utilized the 405, 14 to the 58 route to avoid Friday traffic on my way to Vegas. It works very well for that purpose. Once you get on the 14 it is free sailing. But, I haven't done it towing a boat. The 58 is a two lane highway that is a little bit windy at times but not too bad. Could be a little dangerous. But, I know it is used by NorCal and Central CA people to get to the river so maybe one of them can chime in about towing on that road.
To answer your question about whether it is worth taking the boat out to Havasu if you have to be done by 9am, IMO it depends on whether you are only going there to board or if you enjoy post-boarding chilling and partying. If you only want to board, I would say Naciemento, Mead and even Mojave are better bets. Mead has areas that don't get blown out as quickly. Mojave has more fingers that you can ride all day, even if the middle of the lake is blown out. However, in my experience, it seems like there is always at least one boat in all of them.
I went to Havasu Memorial Day weekend. We barely rode but had an epic time!