A very good question.
It is a very common misconception that trees and plants just always absorb CO2. The Carbon © in CO2 does not just disappear when plants produce Oxygen (O2). Plants use it as material to grow themselves and their fruits. Once they are fully grown, they don’t really absorb any more. So if you burn a tree in a fireplace and grow a new tree in its place, the new tree will eventually re-capture all the CO2 burning the wood released as it grows. This works even better with fast growing plants used for biofuel. The CO2 released by burning biofuel is re-captured when you grow more plants to make more biofuel.
So chopping down a forest to create fields is bad in the short term since it releases and does not recapture the CO2 from the trees, but is sustainable in the long term since you “recycle” the same Carbon.
Almost, yes. It should be close enough as an estimate.
If you want to be precise, one thing you want to be careful about is that not every fuel releases the same amount of energy per kg of CO2. So you should be comparing to the CO2 released by whatever is being replaced by the biofuel (most likely fossil fuel), not the biofuel itself.
Another consideration is how much CO2 is released by the production of the biofuel compared to what it is replacing. Since farming equipment, transportation etc. all could produce CO2.