How would you react to an E2 who "smart mouths" you in formation?

Let him/her know that it is a one way conversation and responses/opinions are not necessary. Then pull them aside afterwards and get down to the nuts and bolts of why they decided to put their two cents in. If there are underlying (off duty or outside) issues then hangle it accordingly. But if it is just insubordination then lay the smack down. Counseling is a must with some sort of paperwork to annotate a possible turd in the work center. But definitely say something tactfully and professionally to the individual in front of their peers to let everyone see that it won't be tolerated and you won't be punked like that by a lower ranking individual. Besides I got more time in the chow hall bathroom than they have in the military.

Front leaning rest for 12 hours

WTF, Oh hell no. You have 4 min. and 30 sec to get that shit done. Now

Take him behind closed doors and find what's bothering him and if nothing is and he just doesn't like the job task given then article 15..

Discipline

Throat punch to ground

Throat punch

Tell him he has become my shadow for the day and when I stop moving he will be in front leaning rest

Spinkicking shit

As this happens in formation in front of everyone else, it is a challenge to authority. Most likely, this Marine already has a reputation, but assuming it is a one off event, there are a couple of options: Call the formation to attention and dismiss, have the PFC stand fast for further discussion. He's more than likely going to end up with NJP "office hours" with the Company Commander, but maybe there is some underlying issue that the young Marine needs help with. Extra duties around the BN spaces on the weekends, etcetera.