By default, it is carried BACK. If you make an election on time, you can make an election to "waive" the carryback to carry it forward, but that time frame is past, so your only option is the 'default' to carry it BACK.
The 'regular' deadline to amend your 2014 tax return is April 15th, 2018, and the 'regular' deadline to amend your 2015 tax return is April 15th, 2019. As long as you do it by then, you can claim the NOL carryback on an amended 2012 and 2013 tax return.
No, there is no limit to how much you can carry back.
Yes, if the NOL can not be fully used on the 2012 and 2013 tax returns, the unused amount is then carried forward until it can be used. For example, the 2014 NOL would be carried to 2012, then the unused amount would be carried to 2013, then the unused amount would be carried to 2015, then 2016, etc..
Yes, a 2016 NOL would be carried to 2014, but then the unused amount would be carried to 2017.
Multiple year NOLs can be quite complicated to calculate and TurboTax does not do that for you. Unless you REALLY know what you are doing I would recommend a tax professional.
