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It could have been several things. A fungus or a root problem sounds most likely as when you cut it back, it started to recover.

Not knowing what kind of mallow you have, I can't say whether its flowering season is over, but when you're having problems with a plant, the first thing to look at is the culture. Again, since I don't know the variety of the plant we're talking about, I can't make anything but general suggestions; you might try cutting back on the water, adding bone meal to the soil, or looking for bugs that consume flower buds.

You likely did not plant Mallow, but it grew as a weed.
Here is a link with more information.

https://www.gardeningknowhow.com/ornamental/flowers/common-mallow/controlling-mallow-weeds.htm

Yes, you can propagate from division, but do this in spring.

Here are some links with more information.

https://www.gardeningknowhow.com/ornamental/water-plants/swamp-hibiscus/swamp-hibiscus-plant-info.htm

http://www.clemson.edu/extension/hgic/plants/landscape/flowers/hgic1179.html

This could be a disease called hollyhock rust. Please see these articles for more information:
https://www.gardeningknowhow.com/ornamental/flowers/hollyhock/hollyhock-rust-in-gardens.htm
http://hort.uwex.edu/articles/hollyhock-rust/

If it's not that, some other possibilities are discussed in this article:
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/answers-to-what-causes-hibiscus-leaves-turning-yellow.htm

Plants in the hibiscus family, like rose mallow, often develop yellow leaves. They tend to be sensitive to the problems discussed in the above article.

Watering is key to the health and growth of you Rose Mallow.
I would increase the watering to keep your plant from wilting.
When the plant wilts, it's all ready stressed.

https://www.gardeningknowhow.com/ornamental/water-plants/swamp-hibiscus/swamp-hibiscus-plant-info.htm