Can Structured Data in an iframe get indexed?

Yes. Google renders pages and in the process picks up the content of any iframe. This includes structured data.

Does json-ld in the head section of an iframe get indexed?

It does. Json-ld script tags get included in the rendered htmls content.

Does Structured Data in an iframe cause rich snippets?

Yes. This page contains FAQPage structured data in each iframe. FAQ Page rich snippets show up for this page.

Does Structured Data in iframes merge with other structured data on the page?

Yes. Each iframe on this page references the same FAQPage id. The testing tools show them as one FAQPage entity.