Difficulty measures chain security, the higher the safer and better.
I'm not sure about this.
If 10 million video cards participate in the mining of a coin, and 55% of the hashrate belongs to one miner. Is this safe for you?
Difficulty shows only the arrival or departure of miners, and miners are interested in profit.
Yes but any coin is subject to this risk. However as the hashrate is much much higher its more expensive to attack and less common to happen.
Its possible for someone to have 51% power but that is like what? 5 million GPUs? It would be very expensive to achieve this. So in general the higher the hashrate the better.