1. For a nikah, a sister who has appointed her own brother as wali (because he is the only one available/qualified) does this brother of hers count as a valid witness to fullfill the criteria as part of the 2 needed witnesses for the nikah?
2. is there any circumstance where a wali can also be one of the witnesses in a nikah according to all 4 madhabs?
Yes, the Wali will suffice as one of the witnesses for the Nikah according to all 4 Madhabs. (Fiqhul Islam vol.9 pg.6562; Syria)
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai