Could you please describe what exactly you're doing to lock the program up? Are you using 1.5.7 or 1.6beta3?

If I understand correctly, this is actually already possible.
You actually can select more than one note via Ctrl+Clicking on them. Or if you want to tag all untagged notes, select "Untagged", then "Edit/Select All", then "Edit/Tag".
Or, from the keyboard [assuming version 1.6 Beta2+]: Ctrl+U, Ctrl+A, F4.

Well the ideas here are more about problems than about solutions. The problem  is basically the same: "There's currently no way to see only tags related to current selection, and only them", so hopefully you can see the reason for merge. Merged, these two ideas will get more votes than being separate.

Ctrl- and Shift- clicks will work the same way. Doubleclicking will just hide/unhide unrelated tags.