A small section of grass is a clump. A wide area of grass is a lawn.
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
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
The adjective is vast. Some synonyms could be extensive, enormous, or mammoth.
Traditionalist
Omnipresent
A walker is a pedestrian.