Can I wear a mini skirt without underwear to Denny's during the evening?

YES!

I think Denny's is pretty safe as they have big booths with big tables so if you sit away from the aisle you will be ok.

The person who you are having dinner with will have no idea you are not wearing panties during the meal as they will only see the top half of you.

The only scary moment for you is going to be when you enter and exit the booth. If you can try to keep an erect posture while exiting the booth sort of standing up before you get to the edge of the table then no one will see.

I think this outing will be pretty low risk.

Do practice getting in and out of the car though if you are with friends.