Yes, I think this behavior is quite appropriate. And I don’t think it’s worth over-optimizing this behavior. If there are no nearby buildings to garrison in, having units stay idle where they are seems like the most appropriate outcome. Players should be responsible for ensuring there are enough nearby buildings for units to take shelter. Otherwise, keeping them idle is actually helpful, since it makes it easier to quickly select them using the idle hotkey.     Short video of this. I don’t see any civilians switching resources after the bell. Not even those who garrison in buildings with rally points set to other resources. ringbell.mp4

Possible. You haven't provided a replay so we're only working with hypothesis. What I described could have mislead you into thinking the units chopping wood were initially farmers. The maximum distance units travel to garrison seems a bit normal. You don't want units to travel half the map if they don't find any hides close enough. Maybe there is way to optimize the behavior however, with better sorting by distance for example. Also the input to ring the alarm is rather limiting...

I’ve noticed that when there are no buildings available for garrison, civilians just stay idle where they are. However, I haven’t verified whether this happens because there are no buildings close enough for them to detect, or because there are no available buildings at all.   What I’m fairly sure about is that units do not switch to gathering a different resource. In other words, if they were farming, they will try to resume farming rather than moving to gather wood.

But, if I understand correctly, that doesn't seem to be the issue @Atrik, the issue is having less people in farms at the end of the alarm because they go to the trees instead.

They can be though, since there is that flood map where the water rises, so maybe it’s possible to change the sky colors in the future?