The effects are connected. Converting more soybeans into fuel means it costs more to buy soy for food - higher incentive to make a new soy field anywhere.
Depends how the subsidy is set up. If it creates a price floor just above the production cost then it just reduces risk for the farmers without incentivizeing them to devote extra field space to soy. Basically the food market gets serviced until the price almost drops below the point where it would be sustainable and the excess is bought up by the Diesel producers (who receive the government subsidy).
By creating a price or demand floor you create an incentive for someone in some country with lax environmental regulations to bulldoze rain-forest to grow soybeans.
Not really, the soybeans used don't have to be grown in the US. So forests in south america are destroyed to make room for soybeans that will be used in the US to make biodiesel.