Elderling Creations Theory (Spoilers through all the books and Dragon Haven)

[Chrischa]

Wizardwood is essentially made from memory stone, isn't it? The dragons swallow the black sand on the beaches of the Rain Wild River, then regurgitate it with their own saliva to create their cocoons. And we know that sand is simply the remains of stone that got carried to the beach from elsewhere by the wind and tides. So, the dragon's cocoons are made from the same black stone.

It seems as if the entire dragon race evolved around this black stone. There must be massive amounts of it deep underground, more then the rare quarries we see in the stories, for an entire race to depend upon it for it's survival. Perhaps this stone used to be the main kind of rock found in this world before that cataclysmic event (which I personally believe was a meteor strike) caused volcanoes to erupt and cover the lands with another kind of earth and stone.

Anyway, I'm drifting off. I think the moment of memory stone and wizardwood's quickening depends on the size of the piece; Kennit's charm awakened with nothing but a saturation of memory because it was so small, whereas a liveship and stone dragons take several complete essences to quicken.

I don't know about Tarman's merging with the new wood... my guess would be the same as yours, that both of his dragons had been dead when the wood was used. Adding new wizardwood woud add fresh memories, and that's one thing that Paragon certainly didn't need.