will a block-chain get so large that the energy to go back and verify the transaction be worth a small percentage of the transaction? As the question implies

also a couple more. who controls the difficult in the computations needed to mine a bit coin? and are there companies who have people mining for them to process their calculations? how does that work.

at the end when all the bit coins are mined. as the economy grows, wont the bit coin keep deflating. increasing its buying power as its just bit coins being passed around in the economy. I can't reason why inflation needs to happen, but it does and always has.