Can someone tell me who built the Shangani old city?

[Grant]

You are assuming Abu Bakr was a direct descendant of Fakr Addin which we don't know.
Also having a paternal arab founder with multiple somali mixture would still make the dinasty as somali as the sheekhaal or the Ajuuraan who have an arab ancestor.

As for the book of Zanj it's an interesting piece of litterature but not very reliable for example it says QUOTE : As to the origin of the Somali, they are without doubt the Arabs, as is said in the book Sabaik al-Dahab, they are descended from the son of son of Bur son of Qaydar son of Ishmael son of Abraham - to their health and blessing

Before DNA testing, Arab ancestor claims were generalized among Somali clans and are not out of place for the given time period. But you make an excellent argument for how some individual Arab men did become Samaale looking and speaking clans. Fakr ad Din could also be such a case, especially as it is attested by a written document supplied by the Reer Faqi/Banu Qahtan and explains their long-term control over the main religious positions in Mogadishu.

Concerning The Book of the Zanj, it seems many Samaales want to discard the whole thing because of the Bantu connections. I ignore what I know is wrong and go with what is confirmed by other sources. Much of it appears to me to have substance.