If an employee has reasonably believes discipline may result from what she says during the interview, she has the right to request representation. Management isn't required to tell employees about their Weingarten rights; it's up to employees to know their rights and request representation.
Lawyers usually aren't involved, either. Representation can be either through a union representative, if your employees are part of a union, or a coworker.
However, a handbook may be considered a contract under certain circumstances, such as when it contains language that may give an employee a reasonable belief that she has guaranteed employment. For this reason, your handbook must be written carefully, and if possible, ask an attorney for help.
An employer doesn't have the power to deny unemployment benefits, only to protest or challenge an employee's eligibility for them. The state unemployment office makes the final decision on benefits. Both the employer and the employee have a right to appeal or challenge the agency's decision if they don't agree with it, however.
Lawful, nondiscriminatory reasons for terminating an at-will employee include:
"At-will" employment also means your employees can leave or quit at any time and for any reason, too.
Probationary employees can normally be discharged at any time within the probationary period with no right to appeal or "fight" the termination. Normally there's right or "entitlement" to continued employment, even in the civil service arena.