it's a moat, btw
Channels make it easier to get out of the water. Before ramps were added to water edges it was nearly impossible for pretty much anything without inate swimming ability to get out, now it's just very hard. All in all, as has been mentioned, it is more practical to use a dry moat, also because when someone falls in you have some way of getting him/her/it out, which is quite hard with a water filled moat (let's not consider magma moats), so
1: yes, most likely. Moving water, and moving water only, has some force to push creatures and objects away. Once water reaches 7/7 it stops 'moving' and sort of teleports to the lower areas. This makes the flow you created much more dangerous even than most water. Add to that that instead of walking up the ramp they have to swim up the ramp, for which, as mentioned, they have no ability or aptitude, while being pushed along by the flow, you should start to see the problem.
2: It's a possibility. The miner also could have been swept away normally and the baby swept from the grasp of the mother who was crossing the channel.
3: No, they will most definately walk along and over the water when it is frozen with no regards for safety
the things mentioned just yet are some of the reasons most people prefer to use moats without water. Those probably have a different name, here they're just called dry moats. Water doesn't add anything in the way of protection and in fact decreases functionality during winter and against swimming creatures, plus creates a hazard for your own dwarves and an obstacle to retrieving items and bodies.