Dear Ivan
Hope you are well and best wishes.
Question 1
"Can captions be added and styled? My captions will have multiple lines, not one string of text. And I'd want, for example, the title to be italic or bold, and the other parts not."
Captions can be added and styled with Lightbox images. Captions are added via a 'Text Editor' so you will have control over how many lines, bold, italic, font, font-size, links etc


If it was really necessary you could add a custom class to the Lightbox element and then your would have complete control over styling it in anyway.
Question 2
"Can captions be added to an image only once, whether they're used in lightbox or not? Or are there separate image captions depending on what kind of display?"
If i understand the question correctly, Lightbox Captions only apply to Images with Lightbox added, other types of captions exists and are available depending on your design. Captions for a Thumbnail Grid for example.
Question 3
"Can viewers on a computer cycle through the images using the arrow keys on their keyboard?"
Yes definitely! πarrow keys work to move from image to image.
Question 4
"When an image is displayed in a lightbox, can viewers pinch-t0-zoom in to the image even further?"
I suppose the 'pinch and zoom' applies to Mobiles only, do you want a pinch and zoom for desktop? (because of lack of fingers on desktop )
Pinch and zoom is not available for desktop. But for Mobile it should be fine. (I think mobile devices have built in zoom regardless of page content).
Hope this helps answer some questions Ivan and have a wonderful day π
Richard