I think you've received some good advice about "it is a defense to prosecution if..." rather than "it is legal to...", and why nobody can provide a yes or no legal answer. But it's also difficult to offer even a "What would you do?" response...not just to your described scenario, but almost any time that question comes up. I know that's not specifically what you asked, but bear with me.
There are a lot of "What would you do?" sorts of scenarios posted on the board. Other than for very scope-limited comments, you don't find many of the more experienced members responding. The reason is that the totality of the situation has to be taken into account before a reasonable response can be offered, and that's
extremely difficult to do in text on the Forum, even to a seemingly detailed after-action type report.
It's a 360-degree world out there in three dimensions and five senses, and situational totality is comprised of umpteen things: emotional state, environmental conditions (obstacles, cover, concealment, retreat routes, witnesses, visibility, footing, etc.), possibility of hidden weapons, number of assailants and positioning, physical characteristics of assailant(s), your own physical condition/capabilities, and many, many more. Your mind can do a pretty amazing job of quickly overlaying and analyzing most of these things but, as the saying goes, "Ya just had to be there."
I'm not saying what-if scenarios are useless. Far from it. I think they're important to think about ahead of time, and to practice. But the "would you draw" posts are almost impossible to respond to because it's inevitable that huge chunks of data will be missing.
But I'd go back to speedsix's comment about: if all you have is a hammer, every problem starts to look like a nail.
The thing that stood out immediately to me is that you're anticipating a large crowd inside a retail establishment. Presumably, even if you get an angry guy wanting to hit you over the head with a display case, the rest of the people in that large crowd are innocent shoppers. That simple situational fact would, IMHO, make a handgun a non-starter in all but the most extreme and unlikely conditions. If you have 65 innocents in an enclosed area in close proximity behind the bad guy, your world of viable options changes dramatically.
The best advice has already been given: if you expect violence, don't be there; have the store rethink the logistics of the deployment; hire professional security, etc. But if none of that will fly, my best suggestion would be to have already discussed this with peer employees, and figure out a way, on your own, that you can work the night in a buddy system. Try to stay positioned close together--but not too close--as much of the time as possible, and make sure each of you keep an eye on the situation happening around the other. There's a reason that bouncers in large clubs, for example, don't work solo: being calmed down by two guys standing apart, flanking you, is more convincing than a lone guy standing in front of you.
There are less-than-lethal options you might consider carrying, but they all have their downsides and there isn't room to go into them here. If you have a streaming OC spray and know how to use it, that's an option. I don't much like aerosol mist OC dispersant in any condition, but certainly not in a crowded retail establishment; and I'd have some Sudecon wipes on me, too. If you use the spray, you're gonna get it on some of those 65 innocent shoppers; a guaranteed story on the local ten o'clock news. You'd also need an OC post-application plan: what do you do once the bad guy is sprayed and is, hopefully, temporarily blinded, keyword being "temporarily."
If the store owner allows you to carry your EDC, more power to her.

But in that over-crowded store environment, I can think of only a few most-extreme and most-unlikely situations that would ever have me even
thinking about reaching for it. Absolutely
nothing is going to go right if you have to unholster among a crowded throng of innocents milling around in a constrained environment.