Welcome to the Q&A Forum
Browse the forum for helpful insights and fresh discussions about all things SEO.
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
I think nobody will link you back with a parked domain with zero content.
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.
I've found the way to identify that who is redirecting a bad repute domain to your's. You can simple check this out via moz pro's Backlinks checker under the option of "301 only".
Now the question is, if someone has redirected a bad domain to mine, what can I do then?
I've searched your indexed pages in Google search and I don't see any tags indexed in Google. I think SEMrush's crawler doesn't count the noindex tags in the page, and it only counts the duplicate pages.
Search it in google: site:www.srgservices.com and tell me if you find any tags' pages indexed.
If you're using Wordpress SEO by yoast then simply go to its settings> Titles & Metas> under the tab of Taxanomies check the radio button of "noindex, follow". see the image:
and if you're not using it then simply add this to your robots.txt file:
Disallow: /tag/
check out this one for sample: http://backlinko.com/robots.txt