So it really wouldn't matter if someone hacked https://app.uniswap.org/ ???

No way to steal funds doing that, hmmm....

Can you turn US dollars or Euros into crypto in decentralized exchanges? As far as I remember you could only trade crypto; one token for another.

That's useful if you have crypto, but how do you get into the field in the first place? As long as people aren't being paid in crypto and can't buy and sell things in crypto there is no way to get into the system without going through someone willing to exchange crypto for fiat. And this someone tends to be a centralized exchange.

No. Did you miss the fact that centralized exchanges set the "market" price of crypto --- even for trades on decentralized exchanges?