We're building a blog on a subdomain of the main site. The main site is on Shopify and the blog will be on wordpress.

I'd like to keep the user experience as simple as possible so I'd like to make the blog look exactly like the main Shopify site.

This means having a menu in the blog that duplicates the Shopify menu.

So is it bad for SEO to have someone click on the 'about us' button in the blog subdomain (blog.mainsite.com) which takes you to the 'about us page' on the main shopify website (mainsite.com)?

Were just doing the same kind of thing with our own website.

Make sure you run the existing site through screaming frog and open site explorer. You want to make sure any incoming links are pointing to relevant pages within the new site. there are a number of plugins in wordpress to handle 301's but I usually start by adding them to the htaccess just because it keeps things a bit tidier.

Have you checked webmaster tools to make sure any broken links are addressed?

Also you'll need to add your Google analytics code to the new site. Maybe try Google tag manager if its a new site.

With posting to multiple article sites, just make sure you post under the same author name. That way you wont be viewed as duplicating content.

The effectiveness of this kind of linking isn't that great though, your better off putting great content on your own site and making sure its shareable on sites like Google plus and Facebook.

Is their any way on your site of adding articles to a page that then pulls in the items you have for sale based on the article? That way at least you would have a relevant landing pages for linking to.