Just a note that NULL columns take up very little space in Postgres. IIRC there is a NULL bitmap for every row. So yes, don't worry and just use columns, if the columns are actually part of your schema.
This is correct. Postgres keeps a bitmap in each row that designates which of the rows are null. Postgres initially allocates one byte for the bitmap so it will work for up to eight columns. Once your table has more than 8 columns, postgres will allocate eights bytes for an additional 64 columns. That means if you have more than 8 columns, you can keep making "optional" fields proper columns until you have a total of 72 columns because null values are completely free.