You only quoted instances where someone was blotted out of the book. I don't remember anywhere in scripture where someone's name is specifically added to it. Therefore, it is still consistent to say that whoever is in that book was in there from the foundation of the world.
Revelation 20:12-15 KJV
(12) And I saw the dead, small and great, stand before God; and the books were opened: and another book was opened, which is the book of life: and the dead were judged out of those things which were written in the books, according to their works.
(13) And the sea gave up the dead which were in it; and death and hell delivered up the dead which were in them: and they were judged every man according to their works.
(14) And death and hell were cast into the lake of fire. This is the second death.
(15) And whosoever was
not found written in the book of life was cast into the lake of fire.
"Was not found written" also is consistent with the writing taking place during this judgment.
I think you are reading the passages as if the the writing of all names was from the foundation of the world. I would say rather that the book
itself is from the foundation of the world, just as the Lamb is slain from the foundation of the world (compare Rev 13:8, 17:8).
And if names can be blotted out, how much more does it stand to reason that names can be written in? Since you have acknowledged that names can be removed, that proves it is not static. Since the book of life is not the only image we are given of salvation, I would point to the gospel to demonstrate that we can be added unto life.
John 3:16-18 KJV
(16) For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
(17) For God sent not his Son into the world to condemn the world; but that the world through him might be saved.
(18) He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.
The default state is "condemned" (which means not written in the book of life) but whosoever believeth in him should have everlasting life (which means being written in the book) ... and we have already agreed that God can blot out or remove names which have been written.