Yes, you can apply Lightbox as default for the previous article. Now to do that you have to make sure that the Lightbox plugin or script you are using is activated on your entire website, once this is done from your side the activated Lightbox will automatically apply to all images within your articles including previous ones.
For the second one, To enlarge wide-format images in the lightbox with the original aspect ratio, this is what you should do: Check the settings of your plugin, most lightbox plugins offer settings to control the image size and aspect ratio, what you can do is you can go and adjust the settings or styles to show your original image without any cutoff.
Still, if the issue remains the same you should check the plugin guidelines regarding images.
