1)Third Party does mean something, " In computer programming, a third-party software component is a reusable software component developed to be either freely distributed or sold by an entity other than the original vendor of the development platform." So anyone creating programs using or ontop of Bitcoin constitutes are a 3rd party, like DarkWallet. Also, all those things form the Bitcoin codebase, and any pull requests still have to be accepted by none other than the Bitcoin Core Developers. You can't just submit any old pull request and get it published immediately without being it overlooked(if that was how it worked, Bitcoin would be destroyed by now).
No, no , no .... you still don't get it. You are completely confused and spreading misinformation. Those 5 Core "Bitcoin core" developers don't have control over other stacks or implementations like libbitcoin and others. Within Alts there is a small team of developers that control the whole coin , within Bitcoin there is one large and mostly used codebase with a large team of developers contributing and peer reviewing and many other codebases that can interact with the Bitcoin blockchain and perform the same actions as Bitcoin core. There is a day and night difference between Bitcoin and any other alt because of this. This means that if a bunch of rogue developers hijacks Bitcoin Core there are other stacks ready to use and already being used right now.
Bitcoin core is only special because it currently has the most developers and users. It has no priveledges or rights above any other software stack that interacts with the bitcoin blockchain!
2)I have never thought of Contributors as software developers. In a way that makes some sense, but it would lead to confusion when trying to explain a Bitcoin Core Developer from a "Core" Contributor on Github. So overall, saying, software developers, is inaccurate or a misleading...
It is only confusing to you because you are obviously not a software developer and are merely reading the words on those pages with no knowledge of how these software projects are actually developed.
Do you see the term "Developer" with Githubs glossary?
https://help.github.com/articles/github-glossary/How about in Permission levels?
https://help.github.com/articles/permission-levels-for-a-user-account-repository/