Download autoreframe_v1.0.0.zip, then in Blender go to Edit > Preferences > Add-ons > drop-down arrow > Install from Disk... and pick the zip (do not unzip it). Tick "AutoReframe - 16:9 to Vertical".

No. It creates a separate AutoReframe camera. Preview and Render switch the resolution and camera only while you use them, then put everything back.

Yes, in the Sequencer: Fill + Blur puts the whole video in the middle with a blurred copy above and below, and Follow Zone shows a part you animate or tie to a tracking point. There is no 3D there, so nothing is re-rendered.