cuz it's only emergency no-garrisonYes, I switched in another unit with about 10HP and let it heal. While it was healing the entire time the Defense process was active (for about 10 turns) and the city strength went from 53 down to 52, then 51 and then back up to 53 without building anything or doing anything in that city except shooting every turn, letting the unit heal and getting attacked every turn. I can confirm that the extra healing works but I don't see how it increases city strength. In another city I tried out the Defense process and it didn't give me any extra defense either, even after letting it "process" for one turn.
afaik the extra defense boost only works when your garrison's strength doesn't boost enough compared to your process's extra strength
they dont stack so whatever is bigger takes the increase