Will the US business suffer from the increased labor costs?
I don't think the employers of labor in the U.S. are going to suffer, the thing in business is about making profits and if the minimum wage is increased, businesses that prolly can't afford to pay all their workers that particular amount will have to make some changes, cutting down on workers is one option, or increasing the cost of the service they render to their customers, one way or another a business must make profit or else it's going to close down. So by and large, the increase is good for the workers, whilst their employers will seek a means to get the most out of this too.
Stupid question:Will the American working class buy more Bitcoin,if they have bigger salaries?

Prolly those whom are already Bitcoin Investors, yes, they could prolly increase their stash, if they earn money, which increases their spare cash, and it could go into buying more Bitcoin.