My best guess on this is either soil deficiency or a fungal disease. But it's weird, I agree with the others, and something I have never seen! Good luck.

That's a fungal disease. It's very common in warm, humid climates, like my state of Virginia. I would move my plants to another bed next year as the fungal spores can live in the soil and splash up onto new leaves in the following years. Yes, over watering can cause it, but so can just plain old rain and humidity. Here's more info to help: https://njaes.rutgers.edu/fs547/

Before the leaves turn yellow, do they get black spots (or brown spots) on them? What is the soil like? I have had Knockout roses die in heavy clay soil and the yellow leaves are typical of blackspot, a fungus.

You may want to open the top of the bags. I just learned that they can get a fungus if you keep them too long in the plastic bags.

No, because the plastic will keep water from percing (draining) into the ground. Instead, use landscape barrier fabric, which you can buy at the garden center. It has tiny holes or openings to allow water to drain through.

You can plant the broken branch to get a new plant. It won't heal itself, but will seal off the end. The main plant should be fine. Here's my tutorial on planting Christmas cactus: https://homegardenjoy.com/site/2018/12/how-to-propagate-a-christmas-cactus.html

Don't! Clover offers good food for bees. It's also perfectly naturally and does not harm lawns.

Winter, before they break from dormancy (grow leaves).

Park Seed has them.

You can use creeping thyme or mosses on sunny paths. Thyme varieties that work well on paths include "Minon" "Pink Chintz" and "Elfin." If the path is in the shade, the USDA recommends two mosses for paths, Plagiomnium (Plagiomnium cuspidatum) and star moss (Atrichum angustatum). These are hardy from USDA zones 2 to 9 so no matter where you live you can grow them. Just keep your paths well watered.

Sounds like soil gnats. Let the pots dry out between waterings and the problem should disappear. You can also use sticky pads, which are available at garden centers. They look like yellow post it notes on a stick. They are covered in a special glue. The yellow color attracts the gnats and they stick to the pads, then you just throw it away.

The only way to do that is to stake and brace the tree in such a way that the ropes will hold it from splitting. The Bradford pear, which is a common ornamental pear, is prone to splitting at the crotch. There are other ornamental varieties which do not split as easily that if you choose to replace your tree, you may wish to consider.