For #1, maybe just an overlay graphic that makes a block appear cracked. If X % of the blocks in a building are cracked, it collapses. Builders or masons or somebody can be tasked to do maintenance / repair. Also, this way you cannot hide in a building forever; you’ll have to come out and fight off the menace unless the mechanics allow repair from the inside.
I like the #2 idea on having a durability. Perhaps the item simply disappears from the unit’s external view when it’s broken, like it’s being carried but not worn.
For #3, another possibility is to have a toggle for the health, similar to my suggestion in a different thread to toggle a morale indicator. You could toggle all units or an individual unit. Maybe this is more of a UI mod, but it would be nice to choose between a health bar or another idea of tinted units, a bubble over the head, a halo/disc under the feet. The simplest I can think for tint is no tint for normal, a yellow/orange tint for damaged, and a red (maybe blinking) tint for nearing death. Someday, it would be really cool if the indicator moved through the whole color spectrum, so you could see shades of colors as the health progresses from green through yellow to red or even black at the very end. I personally really like the idea of blinking color nearing death, but understand some may not like that and what to turn it off.