The names that Martini (Marty) says during shooting practice are all names of various cast and crew members: Mr. (Hagai) Shaham (producer, actor - Handsome Police Officer); Mr. (Jacob Aaron) Estes (director, screenwriter); Ms (Susan and Joan) Johnson (producer and production coordinator); Mr. (Rick) Rosenthal (producer).
After Marty proposes that they bury George's body, Clyde, facing the camera in the foreground of a medium wide shot, turns and starts to walk away while Millie's arms are at her sides (from 59:13 to 59:16). The scene cuts to a closer shot of Millie and Sam with Clyde walking away in the background while Millie is in the foreground with her arms crossed at her chest (from 59:17 to 59:20). In the next shot (from 59:20 to 59:30) Clyde is in the foreground and Millie is in the background with her arms at her sides.
When George is asking Sam if the plan to take him out on the boat was true you can see George in the background moving his lips as if he is speaking but there is no audio to match this (at around 2 mins). This was corrected in the DVD.
In the scene where Marty and Rocky meet Kile and Jasper who have been eating mini-beef tacos in the Quickie-Mart, Jasper is eating from a yellow mustard packet in his left hand (at around 1 min), then a green relish packet in his right hand (at around 5 mins), then a yellow mustard packet in his left hand (at around 8 mins), then a green relish packet in his right hand (at around 13 mins), and then a yellow mustard packet in his left hand (at around 28 mins). Also, the paper food-boat in his other hand switches correspondingly from right to left to right to left to right.
It is never stated and implications aren't apparent throughout the film. There are too many reasons to assume any, it's just not an important factor of the film.
Q
Why did Marty do what he did at the end?
A
Marty knew the others were going to confess, which he wanted no part of. He said he would drive to Mexico, implying a life on the run, thus he began a life of crime, particularly robbing the store for cash.