For no. 2, in. is the abbreviation for inch. You could write either 5'5", or 5 ft. 5 in.
I prefer five feet five inches, but I hear five foot five inches probably more than I hear feet, in common usage. To me, it isn't logical to use the plural for inches and the singular form of foot, so, my opinion is that feet is correct. But, because the usage of the other is so ubiquitous, I'm su