The US will discover, as the British have, that once you have a single supplier then your negotiating position is zero - you HAVE to buy from them
And so you finish up with even bigger cost overruns and kit that just doesn't work
That depends on the nature of the contract. The tanker contract is firm-fixed price. Any cost over runs are borne by the manufacturer, NOT the government. Any equipment that does not meet spec must be redesigned/modified to meet spec at the cost of the manufacturer, not the government. Boeing is going to lose a LOT of money on this contract.