I def agree on pulling back on spices, maybe just 1 vanilla bean and 1 cinnamon stick, and maybe only consider .5 - 1 oz tops of fresh ginger, it's some strong stuff. With any spice in beer, even heavier and darker beer, a little goes a long way. Also agree that these flavors will come through...
I don't think you'll have a problem. I recently made a similar mistake, made a 1 gallon batch of Irish Red and without thinking dosed it with enough Irish moss for 5 gallons. Smelled like you described, seaweed. The smell fermented out and left no off flavors.