Re: Is it good or bad that Core development is virtually controlled by one company?
Mutual destruction, the paragon of maturity 
I don't advocate for this action or repeat these facts to fear monger, but they are important considerations one must consider when
deciding upon what level of consensus one must agree to before activating a fork. It is no surprise that many of the Chinese miners are advocating 90% -95% threshold and a much larger window.
And nodes figure into this how?
The code determines what fits consensus or not. Human beings with Nodes full of BTC that now represent 2 different coins can transfer that BTC from their non-mining node to an exchange or another human which places downward pressure upon one of the coins price.
Well sure, they can create an altcoin, like any other altcoin. How does this effect miners & users with new nodes?
It all depends upon perspective. From their Eyes the big block coin that uses democratic form of governance where a majority can impose changes upon a minority would be the alt where their coin that uses anarchistic forms of consensus would be the true bitcoin. In their eyes the change in political consensus is much larger of a change than the change in the mining algo.