Is it hot throughout the day?

Go in January!

Parts of the hiking trails are shaded and parts not.

Take shade Wear your sunscreen You'll be fine...its the great outdoors not a country club 😉

The area around the reservuor usually stays a few degrees cooler than LA proper and most of the path around the water is shaded

There's shade around the lake, the majority of the hiking trails are not shaded, however, so if the day is hot, the hiking trails will too.

Yes