Good work.

Alternatively, one might construe the argument this way:

1. There will be AI++ (before too long, absent defeaters). [See Chalmers.]
2. If the goals of the AI++ differ significantly from the goals of human civilization, human civilization will be ruined soon after the arrival of AI++.
3. Without a massive effort the goals of the AI++ will differ significantly from the goals of human civilization.
4. Therefore, without a massive effort human civilization will be ruined soon after the arrival of AI++.

But this may be a less useful structure than the more detailed one you propose. My version simply packs more sub-arguments and discussion into each premise.

The premises (in your argument) that I feel least confident about are #1, #2, and #4.