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Hi, Do nothing until Google has rolled this out and confirmed that they are going to go with it. You don't want to make lots of changes if you only have to change them all back again - this could cause you a number of issues. -Andy

It's likely that as the mobile update takes place Google will remove the tagging as it'll be redundant. Nearly everything in their mobile results should be mobile friendly.

Not sure about this specific message you got, but very often messages like this from Webmaster tools only include a few sample links, and then you have to figure it out yourself which the other pages are where the problems occur (Google wouldn't dare to make our lives too easy)

A quick suggestion to identify those sites. Check out their Domain and Page authority Trust flow ( Majesticseo metrics ) Check with their backlink profiles using opensiteexplorer and ahrefs.com. Their backlink profiles must be healthy and coming from quality sites. Check the pages you are going to link to. If they have thousands of outbound links. I will never link back from those kind of pages. Also Donford has suggested very useful points.