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In theory, using the Tombstone of L&D to add Bella to the Goth family, saving and exiting, returning, and using the Resurrect-O-Thingy (from Paranormal career track) to resurrect her SHOULD bring back Bella Goth. It does not work, however as it says 'You don't know any sims who have died.' Two things that don't add up:
1) When Bella came back the first time (through Tombstone of L&D) she had automatic relationships with Cassandra, Mortimer, and Alexander. So technically they should have known her.
2) In Cassandra's memories (after Bella's ghost disappears) she has a memory "Bella Died. This sim is very important to me... BLAH BLAH BLAH" so if she has a memory that an important sim they KNOW died, how can she not KNOW the sim?
I guess 1 + 1 doesn't always equal 2!

if mortimer is not dead try moving the goth family in strangetown and try to find bella Goth

good luck

The Bella in Strangetown is a fake.

Exactly right, the Strangetown Bella is fake.  The real Bella is trapped in Pleasantview.  
Still though, you need to make her have a high enough realtionship with Morty or another sim in the Goth household, then get the ressurector and call the Grim.  She comes back perfect, it's only because she wasn't really made in The Sims 2 to be ressurected, so some of her memories are a bit glitched.  I got her back using the SimPE way, but I have tried getting her back the SimPE-free way.

maybe if you bring them with the tombstone of l and d, kill her, and ressurect her without exiting till you're done, maybe that will do it

If you use the Tombstone of Life & Death Bella will come with no personality or skills. If you leave the lot and come back it say she died. They have memories of her dying but you can't bring her back unless you have SimPe.
That's a real bummer, though I don't really care about the whole Bella situation, I just make a custom Goth family, make them over and play them normally, stuff the science I say :D