The big key is looking at the first 3 ingredients listed on the feed tag. Never get feed that doesn't specifically state what it is in the feed. However if you want a useful tool to make comparisons like this, here is a link to a feed comparison tool - http://www.poultrydvm.com/feeds.php
Yes. Bring him to the vet immediately. Dogs carry lots of bacteria in their mouths which are deadly to chickens. He probably needs antibiotics before sepsis kicks in. Here is a case story about it - http://www.poultrydvm.com/cases/goldie for your reference. And a directory of vets who see...
Add enrichment items to your flock, to help take their minds off pecking each other. Here is a good infographic with ideas - http://www.poultrydvm.com/featured-infographic/environmental-enrichment-for-poultry
Yes you can keep them together. There is no magic ratio, it depends on their personalities. I have two roosters who were introduced later in life together with 5 bantam hens and the hens are fine and so are the roosters.
There is no point in vaccinating them now, its already in the environment. But not all chickens exposed to the virus develop disease. Some are immune to the effects for whatever reason. Here is more info on Mareks - http://www.poultrydvm.com/condition/mareks-disease
It's probably a combination of issues, considering the poor care the bird has been receiving. However, splay leg stands out to me, which is reversible if taken care of now - http://www.duckdvm.com/condition/splay-leg