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When you are collecting backlink from those page, then you said, i got PA benifit, what's DA ? Now qestion to you ? Why you think, Is there no benifit for your DA ?

By the way, when i worked that kind of work, i saw that, its increasing both point. Quality Backlink are always give you good DA & PA.. But remove toxic backlinks. Be patitent.

useful link for increasing domain authority here:

http://www.quicksprout.com/2014/05/19/5-practical-steps-to-improving-your-websites-domain-authority/

Yes you can. But you need to use Network Admin system of Wordpress. I think this link can help you.

Link 01: http://codex.wordpress.org/Network_Admin

Link:02: http://codex.wordpress.org/Create_A_Network

Link 03: http://www.wpbeginner.com/wp-tutorials/how-to-install-and-setup-wordpress-multisite-network/

I checked. You are using WordPress Yoast Plugin. But your theme built with that kind of function. Thats why 2 set meta option you are watching. Use Default Theme option and remove yoast option. I think, It will best practice .

Its not good sign to create unusal link in any post. Internel link is not bad for seo. But if  its related with your others content. you can create. But read some content about internal link. That content will help you to create good internal link.

Check out this content: http://moz.com/learn/seo/internal-link

I am agree with you. Follow this instruction Travis Wiley. I think this instruction help you.

Yap, You are in right way. I mean buying backlink is not good for site. But its very dificult now that create natural backlink. People will always advise you that create good content !  But we know that, natural backlink is very powerful. Actually i used footprint for creating natural backlink. At first you need to track your compititor then try to create backlink there. Besides, you can try this tool for finding natural backlink. Now i am using it. Its helping me.  i think, it will be help you for creating natural backlink.