Just use H1 one time, matt cutts said in a video that he would like to see only 1 H1 tag on a page.

Create content for better user experience, use headings just for your readers, and don't follow these on page tactics very much. Now everyone knows these techniques, and I don't think Google gives higher weight to these things.

Create content for readers

Use H1 as your Page's Heading(Just one time)

Use H2 where you think it's essential

If you've enough time to verify each subdomain in WMT and also removing 400+ domains one by one, then you can go for solution 2. You can't remove subdomain from verified WMT account of main domain, that's why you need to verify each domain.

Adding canonical is a better option, it wouldn't remove all of the demo domains from Google's index rapidly, you have to wait for few months, but you'll be on the safe side.