Flowers are sometimes "deformed"
There are many things that can cause yellow leaves, unfortunately. Some dropping of older, yellow leaves is normal. If it's moving up the shrub, it can be problematic. The deformed flowers happen often and these abnormalities are called "sports." Many factors play into that phenomenom so it's impossible to determine why.
This article may help: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/answers-to-what-causes-hibiscus-leaves-turning-yellow.htm
I live in northern California about 20 minutes from San Francisco. The East Bay.
If you have a hardy hibiscus, then the care is quite easy. It will go dormant outside. If this is a tropical variety, you will have to bring it in for a "semi-dormancy".
This article will help: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/wintering-hibiscus-indoors-winter-care-for-hibiscus.htm
I have a beautiful potted hibiscus and it is very large. I would like to bring it into the house for the winter, but it needs to be cut back. I live in very cold climate, so can't leave it outside. Is it okay to cut it back and if so how much is okay? Any other suggestions would be greatly appreciated. Thank you Marie at jmorgan@paulbunyan.net
Here is a link to an article with tips about how to prune your hibiscus plant.
I've also referenced a second article with tips about how to winter a hibiscus.
Is it 1/3 of the way from the top or bottom? What are nodes -- is it where the leaves grow from the branch? Do I cover the Hibiscus over the winter?
It probably doesn't matter. Either leave a third or cut off a third from the end. Hibiscus is pretty forgiving. The node is where leaves and stems emerge from the branch. Hibiscus are tropical plants and if winters in your area get below freezing, you need to take your hibiscus inside for the winter.
If you are talking about hardy hibiscus, see below.
https://ask.extension.org/questions/173291
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/pruning-perennial-hibiscus.htm
I live in Missouri. My plant is potted and has been on the back deck since I got it this spring.
This article should help:
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-has-white-fungus.htm
what size pots should I use? I have two terra cotta pots that are 17" at the top but narrow down to about 9" and stand about 24" high. I also have wooden barrels that stand approx. 18" high and are approx. 18" around. I am transplanting some hibiscus plants to a new location and would like to pot them if at all possible. Any suggestions will be helpful.
You want to be sure the pot is large enough to accommodate the rootball. I think the 18 inch would be preferable because the other tapers down and might be too narrow for the root ball.
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-container-care.htm
I have a hibiscus plant which has lots of blooms but they do not open anymore than a trumpet shape ?
It probably is stressed due to lack of enough sun, nutrients or water. Review the needs of hibiscus and see if something is coming up short:
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/how-to-care-for-hibiscus-plants.htm