I'm wondering about the limitations in the footer element, if I posted a link to an external site would it render in keeping with the theme? 

Also wondering if it'd be possible to add a link to the social feed in or before the footer? Last x posts, for instance? Or maybe something like a changing daily 'motivational quote' or something?  

 Footer depends on the theme, most themes don't allow adjustments to footer, but they can be customised. Eventually we will have a marketplace for theme customizations.

 

 Adding feeds from other socials is already possible, you can inject any js code to the site in settings. Whether there's a widget for X posts - need to look that up. 

 In the PWA version would it be possible to push alerts when a new image is added?  

 Yes!