How do I reset NPCs in Skyrim?

I'd try the enable and disable console commands.

Make a save game first, and then in the game, open up the console by pressing ~, then click on Mirabelle Ervine. Her RefID (0001C1B9) should show up in the screen. If the RefID is correct (meaning you have selected Mirabelle correctly, and not another object), type disable and then enable.

If it doesn't work, try teleporting her to your location, to try to 'unstuck' her. With Mirabelle still selected in the console (her RefID (0001C1B9) still showing up in the screen when you press ~ [if not, select her again]), type in moveto player.

Try opening the console (` key by default), clicking on the NPC in question, and typing "resetAI". That should reset the NPC's AI and hopefully unstick her. If that doesn't work, you can try "recycleactor", but that may have side effects.