Good afternoon!
From a distance, I can only answer your questions in general terms. As the law contains certain discretionary powers and the immigration authorities deal with them differently, I would also advise you to contact a local advice centre.
1. If the asylum application has been incontestably rejected, the "Aufenthaltsgestattung" formally loses its validity.
2. + 3. Not automatically, you would have to apply for this explicitly. And I think they would have to withdraw their asylum application (but please don't do this without legal advice, maybe they have a chance to be recognised).
4. With§25b, family reunification is only possible in absolutely exceptional situations, which actually never occurs in practice, and this applies all the more to so-called "other family members". Relief is only available for parents of Blue Card holders
5. Not necessarily, §25 para. 1 sentence 3 says: ‘The temporary receipt of social benefits is not detrimental to securing a livelihood for
1. students at a state or state-recognised higher education institution....’
However, the question will then be whether you are entitled to social benefits as a student, so that is another issue.
I would therefore advise you to visit a local migration counselling service, which is confidential and free of charge. You can find the office responsible for you on the following website by entering your place of residence or postcode:
https://bamf-navi.bamf.de/de/Themen/Migrationsberatung/?typ=MBE&
I wish you all the best!
Regards
Anja Kühnel