You can only do that for new registrations by supplying a registration default value while editing the field. Multicheckbox fields, multiselect, etc.. use |*| to delimiter values if you need to have multiple selected at once you'd do so with VALUE_1|*|VALUE_2 for example.

That parameter should work fine for query fields. No, you can't apply it to profile edit. No, there's no workaround. When we refactor our field classes completely we should be able to better handle a default value. For now if you need to apply a default to existing users you could run a query on your database table _comprofiler using phpmyadmin to set their fields value if they don't have one stored.

A by the way thing, any tutorial on how to capitalize on the value layout parameters?

Believe in the CB Primer Book it's described some. In short it lets you replace the output of a field in those respective views. So for example lets say you've a radio field that you want each value to display as an image. That's doabling using the profile value layout and IF substitutions. It replaces the need for having 2 fields to do this as you used to have to use a Custom HTML field in addition to your radio field.