You are required to list all employment, whether employed for a week or a decade.  Failure to do so will result in your termination from further consideration prior to hire, or after hiring, when the background check is done.  In addition, if the hiring agency also conducts preemployment polygraph testing, you will be asked if you falsified any of your security forms.

Falsifying an SF-86 (omission constiitutes falsification) also is a felony;  just read the fine print on the form for the consequences.

depends on the reasons;  employee theft, sexual harassment, or workplace violence definitely are a disqualifier, most other reasons are not.

again, it depends on the hiring agency;  each agency has their own threshold for disqualifying information.