1) You: "If you "collect" watch towers, then you can look for man_made=tower (a tag that's missing here) + tower:type=observation". 
I don't understand. The man_made=tower and tower:type=observation tags are not missing. I had added them in immediately. However, they ARE missing from the list of choices in the POI's. ☺

2) Oooh. Thanks for the tip. Didn't think of that. I always collect them when I spontaneously see them.