WordPress.org

• How can I make regular images appear in a lightbox without [gallery] shortcode?

Just add rel="lightbox" into "a" tag. Here is a sample.

<a href="image.jpg" rel="lightbox" title="this is a caption">
<img src="thumbnail.jpg" alt="" />
</a>

• How can I handle multiple galleries in one page as separate ones?

If you would like to handle galleries separately, add different class names into [gallery]. ex) [gallery class="gallery2"]