• Steemers: What was the motivation for taking over Steem's witness nodes?

It was in part a response because the Steem witnesses made a move to prevent such a takeover by temporarily stopping certain accounts from making transactions. There is speculation, fueled by public statements made by the new owning entity, that the entity had intentions to assimilate Steem with another cryptocurrency.

• Hivers: What was the motivation for the Hive hard fork?

It solved the immediate problem of being under control of a single entity. It also solved the problem of the Genisis stake which was promised to never be used in governance. Essentially decreasing the threat of a centrally managed governance.

• Everyone: How exactly was the takeover / the hard fork carried out? Why was this possible to occur in the first place? Could this happen again in the future? How do you think will the situation evolve?

You seem to ask this question from the standpoint of someone who doesn't quite understand the Steem/Hive blockchain, so I have answered it as such. I mean know offense if you know all of this already.

I think to answer this question you first have to understand the so called "ninja mined" or "Genesis" stake. I attempted to explain it in the following:

A:I made a coin which is governed by those who own the coins.
B:Who owns the coins?
A:I do. Wanna buy some?
B:I can't buy enough to govern
A:I promise that if you buy, I won't use my coins for governance.
B:OK, give me 5%
A:agreed.
A: I sold myself to C
A: I govern your coins

Basically, the creators of Steem mined a large portion of the Steem before anybody knew what it was. This stake was owned under the corporation of Steemit inc. The way the system is set up, each token has the some influence on governance. Therefore having such a large stake essentially gives you control or at least great influence into the code which supports the token. There is a twist though. In order to use your tokens to influence governance, you have to agree to lock it up for 13 weeks. This is called powering up.

Now, the actual takeover happened because Steemit Inc. (with their tokens frozen) was able to convince 3 exchanges to power up their customers funds and put them in power. They did not seem to know that this would lock up their customers funds for 13 weeks. Their customers were not involved in this decision.

This could theoretically happen again on either chain, but Hive has mitigated it by requiring that funds be powered up for 30 days before they can be used for voting. Also, since the Genisis stake is now dedicated towards development, any new "threat" would have to be purchased on the open market.

• Witnesses on both sides: What is your vision for the governance of Steem / Hive? How do you intend to achieve this vision?

Not a witness, but I would like to see more of a push towards decentralization and mitigations for some of the privileges of stake, especially on the social side of the things. Right now, investors with little desire to interact with the social media side of the blockchain are more or less forced to do so in order to gain a share of the reward pool. This leads to actions which are sometimes contrary to the content platform which many people regard these blockchains to be.

• Users on both sides: Which platform do you intend to use onwards (Steem, Hive, or both), and why?

For me, it's both with a greater focus on Hive. Hive because I like the vibe and feel that there is a greater chance for decentralization which enables me to have greater influence (albeit still miniscule) over the governance of the chain.
Steem because a lot of my friends are still here and I still want to support them. There is also a nostalgic pull as I have "grown up" here in a sense.