As you say, chord mode (q) does two things - it affects how the caret advances and it also means that notes which are created or edited will not overwrite other notes. More generally we use chord mode to mean “merge mode” and non-chord mode to mean “overwrite mode”, so there are some subtle differences in how e.g. copy and paste overwrites the existing material that depend on whether chord mode is on or off.
With respect to your second question, I don’t think there’s a simple way of doing that. Perhaps you can filter by pitch?