Skyrim just doesn’t want some NPCs to be killable

Yes! Some NPCs make better spouses, but only as far as utility.

• Spouses open up a "store" and provide you with half of the earnings (depending on the character it could be 100 to 200 gold a day). They will open up a store regardless of where you choose to live. Spouses that previously were shopkeeper NPCs have the stock of their previous shop. Spouses that didn't previously have their own shop open a "pawnbroker" type shop, which can have some nice things in it, like Ebony Ingots and enchanted apparel. Depending on who you marry, you might want/not want their previous shopkeeper stock.
• Spouses that previously provided training continue to provide training after marriage (ex. Farkas). Money you pay them for training temporarily becomes part of the money pool for their buy/sell window too, if they open a shop for you.
• Spouses that could previously be followers/companions will continue to have that option. Not all spouses are able to be followers (ex. Willhelm is not a follower). If you plan to have your spouse follow you, you might want to marry someone who is the class you want. (ex. Farkas is a warrior/tank, Onmund is a mage).
• Spouses that previously provided quests continue to provide those quests. It might be convenient to have that questgiver live in your house instead of their previous location.

One NPC, Angrenor Once-Honored, dies naturally as part of the game unless you marry him.

Other than above, race, appearance, or gender don't make a better/worse spouse. All spouses will live with you, cook, and provide the well-loved resting bonus.

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.