My experience with mummies and canopic jars is that you need to have full canopic jars collection (not just any five canopic jars) in your sim's inventory for them to be able to summon a mummy. First time I tried summoning a mummy I tried to place the canopic jars inside the coffin but they won't go there. Then I realised they just work ok when they are in inventory. You don't lose the collection if the summoning fails, the collection simply gets stuck to your sims inventory. Going to vacation (and maybe returning back home) seems to un-stuck the collection and you get to try summoning again. If the summoning succeeds you lose the collection.
I agree mummies are slow and that Mummy! reaction and items dropping from queue is annoying. My mummy writes and gardens, he's a computer whiz
so he doesn't much object writing. He uses moodlet manager to wipe away that soaked moodlet or to clean himself if he's dirty enough. He doesn't cook because other members of the family does it but if he'd need to do that I'd consider upgrading the stove fireproof instead of improved meal quality.
Little out of topic but I've found out that making friends for my sims is way easy if they
*study one skill point in charisma,
*have Attractive lifetime reward and
*say hello to 25 sims (to do one of the charisma skill challenges called celebrity sim).
Sims they say hello can be anyone like neighbor kid, newspaper deliverer or mail carrier or random sim at Central Park. Once they have done all this every new sim they meet becomes an instant friend. Need to try if this works for mummies too so that they can become leaders of the free world.