Can an employer be required to provide an escort from office to
vehicle?
No, at least, not on the theory articulated in the question.
I can imagine some circumstances where it is conceivable that there might be a duty arising from some other source, like an OSHA regulation applied to a firing range business, or an express contract with the employee (some employers provide an escort as a matter of right in the evenings or at other high risk times, as an employee benefit, especially college and university employers, in part, because they have worker's compensation liability while an employee is still on a large campus, in part because it helps attract employees who may feel vulnerable, and in part because of an attitude that the employer wants its employees to be safe at dangerous times of day and this shows that the employer cares about them), or a court injunction related to a labor-management dispute where the employee is a scab.
There is no such employer duty, but an employer does have strict liability in almost every case (there are some minor exceptions for very small employers and criminal conduct by an employee who is injured when the criminal conduct is clearly outside the scope of duty of the employee) for injuries and death in the course of employment from any cause whatsoever pretty much (including criminal actions of third-parties) which is generally fully insured by worker's compensation insurance.
The exact details of when someone ceases to be at work for worker's compensation/employer liability is buried in case law and regulations (for overtime and minimum wage purposes, the standard is "portal to portal" but workers compensation/employer liability need not be identical, although once you are clearly no longer on the employer's premises and commuting after a day's work is done or before a day's work starts, you are clearly not covered).
But, any place where there is employer liability at all, it would be worker's compensation covered.
Usually, if the employer is required to have worker's compensation but doesn't, the employer likewise has strict liability for the same harms, but the damages that may be awarded are not limited to those that worker's compensation policies would cover.
This leaves the employee with at risk travel between the office and
the vehicle. It seems reasonable as well that as the employer
prohibits the employees self defence, they would be responsible for
the employees defence between office and some safe location (i.e.
vehicle).
This theory pretty much always loses.
An employee walking in an ordinary, non-wartime environment without a firearm is not "at risk" in a meaningful sense, any more than someone who didn't choose to carry a firearm who goes about their daily life (or is prohibited from carrying one due to past conduct such as a felony or a domestic violence protection order or a domestic violence misdemeanor or a condition of parole, probation or bond pending criminal charges).
Also, the employee is not being prohibited from engaging in any kind of self-defense or protective action whatsoever (or from asserting self-defense rights if a firearm is carried contrary to an employer rule) just from carrying a firearm at that particular moment (on pain of losing a job, not forfeiting a legal affirmative defense under criminal statutes), which is one of many means by which a person can protect themselves from crimes.
