indescribable
There is no one-word substitute in English for "son-in-law."
The word you want is florist.
Sportsmanship is a word substitute for spirit of game
It is impossible for someone to write after their death. However, the adjective posthumous refers to an action, comment or honor (or publication of works) done after one's death.
The word metalworking is one word.
eat, use
One possible substitute for "their" is "his" or "her" depending on the gender of the person/people being referred to. Another substitute can be "its" when referring to a non-human object or entity.
The word that works is idiot.
Elevate
Traditionalist
Omnipresent
A walker is a pedestrian.