You may petition for him as an unmarried minor child of LPR.

No, unfortunately you do not appear eligible for adjustment of status.

No, this should not present problems for future processing.

You can inquire again with USCIS but it is fairly common for the processing to take time after obtaining new evidence.

It is not advisable to travel while your U visa is pending.

Yes this is possible in certain settings.

You may do so although results may not be expedited processing.

You may not have a travel document number. Review the instructions for guidance.

Yes you may do so after obtaining German citizenship.

You should change your name and submit I751.