Welcome to the forums, and congratulations on getting so far with your novel!
To answer your questions:
1. Since we can assume your beta readers will not have Storyist themselves, the easiest options are to export to either e-book formats (Kindle, ePub) or to RTF. Storyist does both of these things well. Which you choose would depend on what you want back and which stage of the process you're in. E-books are good for final reading, assuming the beta readers have the right devices, and readers can make notes while reading, although they will have to construct separate files in a word-processing program to send to you. RTF files can be read directly into any version of MIcrosoft Office, Open Office, Pages, or even TextEdit (but be aware that Pages and TextEdit keep your formatting but trash your RTF styles, which creates problems if you try to read the files back into Storyist). The readers can then enter comments directly into the documents for you. Google Docs operates similarly, although personally I don't like how Google Docs handles formatting.
2. When you say "export a Storyist document to a publisher," do you mean an editor at a traditional publishing house? In that case, it's easy: you export the file to RTF, load it into a word processor and resave as .doc or .docx, and send it out. If you mean to a printer like CreateSpace, you have to set up the formatting and the page size in Storyist, then print the file to PDF. It's not difficult, but it assumes you have a basic grasp of styles and how to work with them.
Hope that helps,