Plants as long as they arent like melting, can handle a blackout no problem. When we ship plants, they can go for days without light. Algae gets hit way harder. Do not be afraid to try it. However since you have really reduced the lighting already, i am not sure as i have never tested with ambient light for a few days then blackout. I want to say it will be fine, but since i don't have experience, i cant say so.
Otos are a bit difficult to acclimate and are very shy in low numbers. Also, they will not solve your issues, they are for maintenance and control. In this hobby, controlling algae is better than eradicating it. Amano shrimp will work as well.