Can text content in an HTML5 <canvas> element be indexed by search engines?

Probably not, try this tool http://robot.dabase.com/ to see what a robot typically sees on your site.

If the text is in the HTML, between the <canvas></canvas> tags, then yes. Although I'm unsure of what context the search engine would see that text in.

If the text is drawn to the canvas then no.

Don't really understand the question, do you mean

 <canvas>Some text</canvas>

? That would be silly for reasons which have nothing to do with search engines. The <canvas> tag is for rendering using JavaScript, so if you are asking about that, your question is sort of equivalent to whether search engines can see text written by JavaScript.