Yes. This is the way to install extensions if you’ve downloaded the .rbz to your computer.
No. Just dragging the .rbz into a folder does not extract its contents.
That ought to be fixed so it works.
.rbz files could be renamed to .zip, the contents extracted and copied into the appropriate Plugins directory but this method is prone to problems unless you get everything done exactly right.
You could install the Sketchucation Extension Store and use its installer to automatically install extensions such as those from TIG and Fredo6.
Not only does it make it easier to install extensions from Sketchucation, it will help you keep them up to date and create a bundle of SCF extensions that can be auto installed when you upgrade to a new version or move to a new computer.