What is the term for accusing a person of mischief through invented, twisting and changeable terminology?
Solution 1:
"Jargonaut" would be a possible neologism.
Solution 2:
The art of winning an argument through the use of deliberately confusing language is casuistry. A person who engages in it is a casuist.
Solution 3:
Since Humpty Dumpty in Lewis Caroll's Through the Looking Glass says
When I use a word,' Humpty Dumpty said in rather a scornful tone, 'it means just what I choose it to mean — neither more nor less.
I guess you could call them that, although it doesn't exactly cover everything you ask for.