1) Is it possible to publish a game made in C3 on STEAM?
2) To publish on STEAM I need to use NWJS?
Yes (BUT I have seen an electron version floating around, not sure about the differences)
3) Is the process of publishing on STEAM very painful?
I don´t know that one, can´t be worse than publishing on mobile. Keep in mind that there is a one time fee too (100$ iirc)
4) Should all files generated with NWJS be placed in a single executable file?
Not that I´m aware of.