Welcome to Steemit! Hope you will enjoy your time here. I think you'll find lots of techies in the community who share your interests (I'm one of them, a long-time C++ programmer and cryptocurrency enthusiast myself). I can't answer all your questions, but I'll take a shot at a couple of them:

a regulator would have a tough(er) time shutting down a blockchain-based currency like bitcoin, but wouldn’t shutting down the blogging piece of Steemit or a third party Ethereum DAPP be no different than shutting down a Twitter or other centralized tech platform?

You are right in that the UI for decentralized apps is one point of centralization that could be a weak point for them. But the back-end of such apps (the blockchain itself) can't be shutdown. If the Steemit web site were shutdown, somebody could simply copy the web site's code and put it up on another server. Or another service could be created by another group of developers, call it Steemit Reborn, re-inventing the project but using the already existing data on the Steem blockchain. Trying to shutdown a dapp is like playing whack-a-mole - another version of it will simply pop back up in another place, utilizing the same back-end as before. In the case of Ethereum dapps especially, most of the code for the app itself is stored on the blockchain, and you just need a thin layer of JavaScript & HTML to provide an interface to it.

Is there no way to store data like this via blockchain technology and completely decentralize it?

Yes, technologies are being developed to do this. I suggest researching the IPFS (Inter Planetary File System) for starters.

In what ways can developers integrate Steemit/Steem with existing apps built in other more traditional programming languages to benefit those apps?

I don't have any direct experience with blockchain programming, but my understanding is you typically have to run a node or connect to one in some way to be part of the network, then you interface with the blockchain by means of some API. There is a JavaScript API for Ethereum. I believe Steemit's API is written in Python. Search for articles on Piston and read all the blog posts by @xeroc

Hope that helps a bit, and good luck with your project!