That's a nice, beautifully-rendered project. I winced when you mentioned copyright. On my sites (eg GiveThemLife.com), I try to avoid copywritten material in on-site content. I can't tell people it's free for any use if my dependencies might not be. That God gave us His Word as a free gift, but His children restrict it, just bothers me.

I'm so grateful for MPJ and others who made the World English Bible. It's good enough most of the time. If not, I sometimes customize the passage by mixing it with synonyms from other, public-domain translations. Even the dynamic translations, like Weymouth, are good for mining words. Many are on studybible.info, e-SWORD modules, and even on eBay in book form.

Far as your tool, I don't see how you could merge copywritten content into the main interface. However, you might add a button or menu action for each verse that generates a pop-up to an external page with many translations. BibleHub is good for that. One side effect is that your site causes people to bump into other content that may help them.

Re licensing. I am curious how much they wanted to charge you. Especially for ESV. For one translation, WEB's author was quoted $10,000 plus $10 a copy for use in free software.