emmaj

All this info is so great @brandon
In case Squarespace is keeping an eye on this, the most annoying thing now for my client is that they will have to go from a Personal to a Business plan to allow for the JS customisation. 
Adding captions to galleries is free for a Personal pricing plan in 7.0 ... hopefully the captions will be also added as default for 7.1.
 
 

All this info is so great @brandon
In case Squarespace is keeping an eye on this, the most annoying thing now for my client is that they will have to go from a Personal to a Business plan to allow for the JS customisation. 
Adding captions to galleries is free for a Personal pricing plan in 7.0 ... hopefully the captions will be also added as default for 7.1.
 
 

Hi @emmaj.
A1: There are a couple ways to do this. The JavaScript is setup so that you can pass in your own CSS selector. That allows you to set which galleries get descriptions (by default, all of them do). In your case, it sounds like you only want to select gallery lightboxes. So replace this line in the JavaScript:
addGalleryItemDescriptions(); with
addGalleryItemDescriptions(".gallery-lightbox"); An alternative way (instead of the above) would be to simply leave the JavaScript alone (so all galleries would still have descriptions) but then hide it on certain galleries using CSS. You'd use your browser's inspector/developer tools to find the correct selectors and write the CSS. Something like this might work:
section.gallery-section .gallery-item-description { display: none !important; }  
A2:
.gallery-item-description { text-align: left; }  
A3:
This would technically be possible by using the image file name as the title. You'd be sure you name your file "This is my title.jpg", then the JavaScript component of the code I wrote could be amended to add that as the title (minus the file extension ".jpg".). That'd take a fair bit of additional custom development time, but it is possible.