There is no difference between 2 and 3. You either shoot progressive or interlaced – or some odd flavor of interlaced that pretends to look like progressive (some Sony cameras) but is still interlaced.
It would help if you specified the camera and the exact mode. I don’t know that there are any cameras shooting 1280X720 in an interlaced manner. I do believe that most 1280X720 is progressive, if not all of it.
As for fields and frames….
Frames are made up of two fields taken 1/60th of a second apart in interlaced footage. To deinterlace you can throw out one field or merge them together, of any number of different schemes. Progressive is nothing more, really, than one field taken for a longer duration.
Interlaced can have jaggies but look better on an interlaced TV, and Progressive can smear a bit. Both have their place but I believe that Interlaced will disappear completely when we are all shooting 1080P.
In any case, when you say you have interlaced footage at 1280X720, I am a little surprised. Can you please be more specific as to how that was accomplished.
Steven
https://www.stevengotz.com