Termination of rental contract, new address, child benefit for builders

I am currently preparing the termination of my rental agreement. I have a 3-month notice period. We are currently building, and the screed has been installed in the house since the beginning of the month. This means the further work inside is manageable. Now I have to, for example, inform the landlord of a new address for the service charge statement, etc.

1:
Can I already specify the new address of the house? (I will then set up the mailbox).
2:
Will official mail already be sent to the new address if I am still registered at the old address?
3:
We can also temporarily stay with family members. Would it be advisable to register there for the transitional period (e.g., because of official mail)?
4:
Could this possibly affect the application for Baukindergeld?

Last but not least, a question:
5:
House handover by the general contractor: This is just the date of the "reversal of the burden of proof," right? But you can move in before that, can't you?

5: Handover of the house by the general contractor: This is just the date of the "reversal of the burden of proof," right? But you can move in earlier, can't you?

If I remember correctly, moving in counts as (defect-free) acceptance, so be careful! But I don't have it exactly in mind whether there are still any "ifs and buts."

Hello, the post office offers to forward your mail for a fee ...

Hello, the post office offers to forward your mail for a fee ...

It's called a mail forwarding order and costs about 20 euros for one year.

1. Why not? We have also been having the mail sent to the new house for almost a year, where we are not yet living. 2. and 3. What official mail are you expecting? See above: You can apply for mail forwarding. 4. I don't know 5. I once heard that moving in is equivalent to acceptance, so not recommended. Maybe you can arrange something individually with your construction company?